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B1 noun #3 most common 8 min read

intruder

Top 3

Practice it Examples Notes & Tips intruder in 30 Seconds

Explanation intruder in your Level:

An intruder is a bad person who goes into a house or a building when they are not allowed. Imagine you are at home and you see a person you do not know inside your living room. That person is an intruder. They did not knock on the door, and you did not say 'come in.' We use this word for people who break the rules about where they can go. Most of the time, an intruder is someone we are afraid of because they might want to steal something. You might hear this word in a story about a hero catching a bad guy. It is a very important word for safety. If you see an intruder, you should call for help. You can also use it for animals. If a big bear comes into your tent while you are camping, the bear is an intruder. It is in a place where it should not be. Remember: an intruder is someone who is in a place without permission. They are not a friend or a guest.

At the A2 level, you should know that an intruder is someone who enters a place without permission. This place is usually private, like a home, an office, or a fenced yard. People use alarms and locks to keep intruders out. If an intruder comes into a house, the alarm will make a loud noise. This word is more serious than 'stranger.' A stranger is just someone you don't know, but an intruder is someone who is doing something wrong by being there. You can use 'intruder' in sentences like 'The dog barked at the intruder' or 'The police caught the intruder.' It is also used in computer science. If a person tries to look at your private emails without your password, they are a digital intruder. It is a useful word for talking about security and rules. When you use this word, you are saying that someone has crossed a line they should not have crossed.

For B1 learners, an intruder is defined as a person who enters a building or grounds without permission, often with the intent to commit a crime or cause a disturbance. It is a key term in discussions about security, law, and privacy. You will often see it in news reports, such as 'The intruder was apprehended by security guards.' It's important to distinguish an intruder from a 'trespasser.' While both enter without permission, 'intruder' often implies a more threatening or active presence, whereas 'trespasser' is a more formal legal term for someone on private land. You might also use the word metaphorically. For instance, if you are having a very private conversation with a friend and someone else starts listening, you might feel like they are an intruder in your private moment. In the world of technology, an intruder is a person who gains unauthorized access to a network. This word helps you describe situations where boundaries—whether physical, social, or digital—have been violated.

At the B2 level, 'intruder' is understood as a person who encroaches on a space or situation where they are not welcome. The term carries a strong connotation of violation. In a legal or security context, an intruder is someone who bypasses barriers to enter a restricted area. This could range from a simple trespasser to a sophisticated burglar. However, B2 speakers should also be comfortable using the word in more abstract contexts. For example, an invasive species in an ecosystem is often referred to as an 'ecological intruder' because it disrupts the natural balance. In literature and psychology, an intruder might be a thought or a feeling that 'intrudes' upon one's peace of mind. The word is often paired with specific verbs like 'detect,' 'repel,' or 'intercept.' Understanding the nuance between 'intruder' and 'interloper' is also useful; an interloper is someone who involves themselves in others' affairs, while an intruder specifically emphasizes the act of entering a space. It is a versatile noun essential for discussing security, ethics, and social boundaries.

For C1 students, 'intruder' represents a sophisticated concept of boundary violation. While the primary definition remains someone who enters a physical space without authorization, C1 usage extends into complex metaphorical and technical territories. In cybersecurity, 'intruder detection' is a major field of study, involving the identification of anomalous patterns that suggest a breach. In a sociological sense, an intruder can be a person who enters a community or social stratum where they are perceived as not belonging, often triggering defensive mechanisms from the 'in-group.' The word can also be used in medical contexts, such as an 'intruder' cell or pathogen that the immune system must identify and destroy. C1 learners should appreciate the word's Latin roots—'intrudere,' meaning to thrust in—which perfectly captures the forceful, uninvited nature of the act. Using 'intruder' instead of 'visitor' or 'guest' is a deliberate choice to frame a person's presence as a negative or disruptive event. It is a powerful tool for precise description in academic, legal, and literary writing.

At the C2 level, 'intruder' is a nuanced term used to explore the philosophy of space, privacy, and belonging. It describes an entity—human, digital, or abstract—that violates the integrity of a defined system. C2 speakers use the word to discuss the 'intruder' in a psychological sense, such as intrusive thoughts that disrupt the psyche's equilibrium. In political discourse, the term might be used to describe foreign influences or 'intruder' ideologies that are perceived to threaten national identity. The word is also central to the 'home invasion' subgenre of film and literature, where the intruder serves as a catalyst for exploring themes of vulnerability and the breakdown of the domestic sanctuary. A C2 learner should be able to use the word with precision, distinguishing it from 'infiltrator' (which implies a hidden, long-term presence) or 'encroacher' (which implies a gradual taking over of space). Whether discussing the biological mechanisms of an immune response or the legal complexities of property rights, 'intruder' remains a vital term for describing the tension between an established interior and a threatening exterior.

Practice it Examples Notes & Tips

intruder in 30 Seconds

An intruder is someone who enters a place without permission, often implying a breach of security or a threat to privacy and safety.
The word is commonly used in crime reporting, cybersecurity, and nature documentaries to describe uninvited people, hackers, or territorial animals.
While often associated with criminals like burglars, an intruder can be anyone—including a lost person—who lacks the right to be in a specific space.
In a metaphorical sense, an intruder can be an unwelcome person in a social group or an intrusive thought in one's mind.

The word intruder is a noun that describes a person who enters a place without an invitation, permission, or legal authorization. At its core, the term is built upon the concept of 'intrusion,' which is the act of pushing oneself into a space where one does not belong. While the word often brings to mind a criminal breaking into a home at night, its application is much broader, covering physical, digital, and even ecological contexts. In a physical sense, an intruder is someone who bypasses security measures—like locks, fences, or guards—to gain access to private property. This could be a burglar looking to steal, a curious teenager trespassing on abandoned land, or even a political protester entering a restricted government building. The psychological weight of the word is significant; calling someone an 'intruder' rather than a 'visitor' or 'guest' immediately frames their presence as a violation of safety and privacy.

Physical Intruder: A person who physically crosses a boundary into a home, office, or restricted zone without right. Example: A masked man found in a warehouse after hours.

Digital Intruder: In the realm of cybersecurity, an intruder is a hacker or unauthorized user who gains access to a computer network or database to steal data or cause damage.

The security alarm was triggered the moment the intruder stepped through the broken window.

In social settings, the word can be used metaphorically. If you walk into a private conversation where you weren't invited, you might say, 'I'm sorry, I don't mean to be an intruder,' though 'interloper' or 'gatecrasher' might be more specific depending on the level of formality. In biology, we sometimes refer to 'intruder' species—invasive plants or animals that enter an ecosystem where they don't naturally belong and begin to disrupt the local balance. The word carries a sense of 'forcing' one's way in, derived from the Latin 'intrudere,' where 'in-' means into and 'trudere' means to thrust or push.

The software is designed to detect any intruder attempting to bypass the firewall.

Social Intruder: Someone who enters a private social circle or conversation without being invited, often making others feel uncomfortable.

She felt like an intruder at the family reunion since she had only just met the groom.

The dog barked fiercely to warn the intruder away from the garden gate.

Police are searching for the intruder who was caught on the surveillance footage.

Using the word intruder correctly involves understanding its role as a count noun. You can have one intruder or many intruders. It is frequently paired with verbs that describe detection, movement, or removal. For example, one might 'spot,' 'detect,' 'confront,' or 'expel' an intruder. Adjectives often provide more detail about the nature of the intrusion: a 'silent' intruder suggests stealth, while a 'violent' intruder suggests immediate physical danger. In a more technical or formal context, you might see 'unauthorized intruder.'

Verb Pairings: Common verbs include: detect, identify, apprehend, ward off, and intercept. Example: 'The motion sensors are calibrated to detect any intruder.'

The homeowner managed to scare off the intruder by turning on all the outdoor lights.

When writing about intruders, the focus is often on the breach of a boundary. Therefore, sentences often include prepositional phrases like 'into the house,' 'onto the property,' or 'within the network.'

Adjective Modifiers: Common adjectives: unwanted, masked, unknown, potential, and persistent. Example: 'The persistent intruder tried several different doors before giving up.'

In literature, an intruder can be a metaphorical force. A character might feel like an intruder in their own home if they no longer feel they belong there. This usage expands the word from a simple legal definition to a deep emotional state.

Despite being invited, he felt like an intruder in the high-society gala.

Collective Use: While usually referring to individuals, it can refer to groups. Example: 'The border was reinforced to prevent intruders from crossing illegally.'

Cybersecurity experts are constantly looking for ways to block a digital intruder.

The intruder left behind several footprints in the muddy garden.

She was startled by the intruder's reflection in the mirror.

You will encounter the word intruder in several distinct environments. The most common is in news reporting and crime dramas. When a person enters a residence illegally, news anchors and police officers use 'intruder' as a neutral but serious descriptor before a specific charge like 'burglary' or 'trespassing' is proven. It is a staple of the 'home invasion' thriller genre in movies, where the tension relies on an unknown intruder lurking in the shadows.

In the News: 'Police are investigating reports of an intruder at the local elementary school over the weekend.'

The alarm system is designed to alert the authorities of any intruder within seconds.

Another major area is the tech industry. Cybersecurity professionals use 'intruder detection systems' (IDS) to monitor networks for malicious activity. Here, the 'intruder' is not a physical person but a set of unauthorized data packets or a remote hacker. This usage highlights the word's flexibility in describing any violation of a secure boundary. In nature documentaries, narrators use the word to describe territorial disputes. If a male lion enters the territory of another pride, he is described as an intruder. This emphasizes the biological instinct to protect one's space.

In Nature: 'The alpha wolf growled at the intruder, a younger male from a neighboring pack.'

The castle's high walls were built specifically to keep out any foreign intruder.

You might also hear it in social commentary. When a neighborhood undergoes rapid gentrification, long-time residents might view newcomers as intruders who are changing the local culture without permission. This is a more figurative and emotionally charged use of the word.

The intruder was quickly subdued by the security guards before reaching the stage.

Every time the doorbell rang, the cat hid, fearing a potential intruder.

The silent intruder moved through the dark hallway with practiced ease.

One of the most frequent mistakes learners make is using intruder when they actually mean 'burglar.' While all burglars are intruders (because they enter without permission), not all intruders are burglars. A burglar specifically enters a building with the intent to commit a crime, usually theft. An intruder might simply be someone who is lost, a trespasser taking a shortcut, or someone seeking shelter. Using 'intruder' is safer if you don't know the person's intent.

Intruder vs. Burglar: An intruder is anyone who shouldn't be there. A burglar is an intruder who wants to steal things.

Don't call him a burglar yet; we only know he was an intruder who entered through the back gate.

Another mistake is confusing 'intruder' with 'stranger.' A stranger is simply someone you do not know. A stranger can be a guest at a party or a person walking down the street. They only become an intruder if they enter your private space without being allowed. Learners also sometimes struggle with the preposition. It is 'intruder in' or 'intruder into,' not 'intruder at' (unless referring to a specific event).

Preposition Error: Incorrect: 'The intruder at the house.' Correct: 'The intruder in the house' or 'The intruder who entered the house.'

The intruder was not a thief, but a confused neighbor with dementia.

Finally, avoid using 'intruder' for people who have a right to be there but are annoying. For example, an annoying coworker isn't an 'intruder' in the office, though they might be an 'interruption.' The word 'intruder' implies a violation of a boundary that should be closed.

The motion-activated floodlights are a great way to deter any intruder.

The intruder fled the scene before the police arrived.

He felt like an intruder when he accidentally walked into the wrong hospital room.

While intruder is a versatile word, several synonyms offer more specific shades of meaning. Understanding these can help you be more precise in your writing and speaking.

Trespasser: A legal term for someone who enters land or property without permission. It is less 'scary' than intruder and often refers to people walking on private fields or lots.

Interloper: Someone who becomes involved in a place or situation where they are not wanted or are considered not to belong. Often used in social or business contexts.

The corporate world viewed the young startup as an intruder in a well-established market.

Gatecrasher: Specifically refers to someone who attends a party or event without an invitation. It is much more informal and less threatening than 'intruder.'

Infiltrator: Someone who enters an organization or place secretly, usually to gain information or cause damage from the inside. Common in spy movies.

The spy acted as an intruder to gather intelligence on the enemy's plans.

The garden was protected by a fence to keep out any animal intruder.

He was labeled an intruder after he was found wandering the private beach.

The intruder's presence was only discovered when the dog started growling at the closet door.

How Formal Is It?

Formal

"The unauthorized intruder was apprehended by the facility's security personnel."

Neutral

"The police are looking for the intruder who broke into the office last night."

Informal

"Some random intruder tried to get into the party, but we kicked him out."

Child friendly

"The brave dog barked at the intruder to keep the family safe."

Slang

"We had a gatecrasher last night; some dude just walked in like he owned the place."

Fun Fact

The root 'trudere' (to push) is also found in other English words like 'extrude' (to push out), 'protrude' (to push forward), and 'obtrude' (to push something in the way).

Pronunciation Guide

UK /ɪnˈtruː.dər/

US /ɪnˈtruː.dɚ/

The primary stress is on the second syllable: in-TRU-der.

Rhymes With

computer commuter tutor shooter scooter suitor neuter looter

Common Errors

Pronouncing the 'u' as a short 'u' (like in 'up') instead of a long 'oo'.
Stressing the first syllable (IN-tru-der) instead of the second.
Confusing the 'tr' sound with a 'ch' sound.
Omitting the 'n' sound in the first syllable.
Pronouncing the final 'er' too strongly in non-rhotic accents.

Difficulty Rating

Reading 3/5

The word is common in news and fiction, making it easy to recognize.

Writing 4/5

Requires understanding of prepositions and the difference between similar terms like 'burglar'.

Speaking 3/5

Pronunciation is straightforward once the stress is learned.

Listening 3/5

Easily identified in security contexts and crime stories.

What to Learn Next

Prerequisites

enter permission private security stranger

Learn Next

trespass violation boundary apprehend malicious

Advanced

interloper encroachment infiltration breach surveillance

Grammar to Know

Using 'an' before words starting with a vowel sound.

An intruder (starts with 'i').

Relative pronouns for people ('who').

The intruder who was caught.

Passive voice for unknown actors.

The window was broken by an intruder.

Possessive nouns with 's.

The intruder's footprints.

Prepositions of place (in, into).

An intruder in the house.

Examples by Level

The dog barked at the intruder.

Anjing itu menggonggong pada penyusup.

Subject (The dog) + Verb (barked) + Preposition (at) + Object (the intruder).

Is there an intruder in the house?

Apakah ada penyusup di dalam rumah?

Question form using 'Is there'.

The intruder ran away.

Penyusup itu melarikan diri.

Past tense of 'run' is 'ran'.

We saw an intruder on the camera.

Kami melihat penyusup di kamera.

Use 'an' before 'intruder' because it starts with a vowel.

The intruder was wearing a mask.

Penyusup itu mengenakan topeng.

Past continuous tense: was + wearing.

Do not let the intruder come inside.

Jangan biarkan penyusup itu masuk.

Imperative sentence (command).

The police caught the intruder.

Polisi menangkap penyusup itu.

Past tense of 'catch' is 'caught'.

The intruder was very quiet.

Penyusup itu sangat diam.

Adjective 'quiet' describes the intruder.

The alarm went off when the intruder opened the window.

Alarm berbunyi ketika penyusup membuka jendela.

Phrasal verb 'went off' means started ringing.

She felt scared because she thought there was an intruder.

Dia merasa takut karena dia pikir ada penyusup.

Using 'because' to give a reason.

The intruder tried to steal the computer.

Penyusup itu mencoba mencuri komputer.

Infinitive 'to steal' after the verb 'tried'.

He called 911 to report an intruder.

Dia menelepon 911 untuk melaporkan penyusup.

Purpose expressed with 'to report'.

The intruder was not able to get past the gate.

Penyusup itu tidak bisa melewati gerbang.

'Was not able to' is a formal way to say 'could not'.

The security guard spotted the intruder on the roof.

Penjaga keamanan melihat penyusup di atap.

Verb 'spotted' means saw suddenly.

No one knew how the intruder entered the building.

Tidak ada yang tahu bagaimana penyusup itu masuk ke gedung.

Indirect question using 'how'.

The intruder left a footprint in the mud.

Penyusup itu meninggalkan jejak kaki di lumpur.

Countable noun 'footprint'.

The homeowner confronted the intruder in the kitchen.

Pemilik rumah menghadapi penyusup di dapur.

The verb 'confronted' implies a direct meeting.

The intruder was identified as a local teenager.

Penyusup itu diidentifikasi sebagai remaja setempat.

Passive voice: 'was identified'.

They installed a new system to detect any potential intruder.

Mereka memasang sistem baru untuk mendeteksi penyusup potensial.

Adjective 'potential' describes the possibility of an intruder.

The intruder didn't manage to take anything valuable.

Penyusup itu tidak berhasil mengambil barang berharga apa pun.

'Didn't manage to' means failed to.

She felt like an intruder at her ex-boyfriend's party.

Dia merasa seperti penyusup di pesta mantan pacarnya.

Metaphorical use of 'intruder'.

The intruder was caught red-handed by the police.

Penyusup itu tertangkap basah oleh polisi.

Idiom 'red-handed' means while committing a crime.

The fence was high enough to keep out most intruders.

Pagar itu cukup tinggi untuk menghalangi sebagian besar penyusup.

Adjective + 'enough' + infinitive.

The intruder entered through an unlocked back door.

Penyusup itu masuk melalui pintu belakang yang tidak terkunci.

Preposition 'through' indicates the path.

The software is designed to alert the admin if an intruder attempts to access the server.

Perangkat lunak ini dirancang untuk memperingatkan admin jika penyusup mencoba mengakses server.

Conditional sentence with 'if'.

The intruder's motives remain unclear to the investigators.

Motif penyusup tetap tidak jelas bagi para penyelidik.

Possessive form: 'intruder's'.

He was charged with trespassing after being found as an intruder on the military base.

Dia didakwa melakukan pelanggaran wilayah setelah ditemukan sebagai penyusup di pangkalan militer.

Gerund 'being found' after the preposition 'after'.

The cat treated every guest as an unwelcome intruder.

Kucing itu memperlakukan setiap tamu sebagai penyusup yang tidak diinginkan.

Simile using 'as'.

Security measures were tightened to prevent any further intruders.

Tindakan keamanan diperketat untuk mencegah penyusup lebih lanjut.

Passive voice: 'were tightened'.

The intruder managed to bypass the sophisticated security protocols.

Penyusup itu berhasil melewati protokol keamanan yang canggih.

Verb 'bypass' means to go around.

Her presence felt like an intrusion, making everyone feel like an intruder in their own home.

Kehadirannya terasa seperti gangguan, membuat semua orang merasa seperti penyusup di rumah mereka sendiri.

Using 'intrusion' (noun) and 'intruder' (noun) together.

The intruder fled the scene before any witnesses could identify him.

Penyusup itu melarikan diri dari tempat kejadian sebelum ada saksi yang bisa mengidentifikasinya.

Conjunction 'before' connecting two clauses.

The intruder exploited a vulnerability in the network's firewall.

Penyusup itu mengeksploitasi kerentanan dalam firewall jaringan.

Technical verb 'exploited'.

The narrative depicts the protagonist as an intruder in a world of privilege.

Narasi tersebut menggambarkan protagonis sebagai penyusup di dunia yang penuh hak istimewa.

Literary analysis context.

The immune system identifies the virus as an intruder and begins to produce antibodies.

Sistem kekebalan tubuh mengidentifikasi virus sebagai penyusup dan mulai memproduksi antibodi.

Biological metaphor.

Despite the peace treaty, any soldier crossing the border was viewed as an intruder.

Meskipun ada perjanjian damai, setiap tentara yang melintasi perbatasan dianggap sebagai penyusup.

Participle phrase 'crossing the border' modifying 'soldier'.

The intruder's stealth suggested that they had professional training.

Kerahasiaan penyusup menunjukkan bahwa mereka memiliki pelatihan profesional.

Noun 'stealth' as the subject.

He felt like a psychological intruder, constantly analyzing his friends' private thoughts.

Dia merasa seperti penyusup psikologis, terus-menerus menganalisis pikiran pribadi teman-temannya.

Abstract usage.

The intruder was apprehended within minutes of the breach.

Penyusup itu ditangkap dalam hitungan menit setelah pelanggaran.

Formal verb 'apprehended'.

The poem explores the theme of death as the ultimate intruder.

Puisi itu mengeksplorasi tema kematian sebagai penyusup utama.

Philosophical context.

The ontological status of the intruder in the film remains ambiguous until the final scene.

Status ontologis penyusup dalam film tersebut tetap ambigu hingga adegan terakhir.

High-level academic vocabulary ('ontological', 'ambiguous').

The invasive species acts as a biological intruder, decimating the indigenous flora.

Spesies invasif bertindak sebagai penyusup biologis, memusnahkan flora asli.

Scientific context.

Her intrusive thoughts were like a mental intruder that she could not evict.

Pikiran-pikiran intrusifnya seperti penyusup mental yang tidak bisa dia usir.

Psychological metaphor.

The hacker was a digital intruder who managed to exfiltrate gigabytes of sensitive data.

Hacker tersebut adalah penyusup digital yang berhasil mengeksfiltrasi gigabyte data sensitif.

Cybersecurity jargon ('exfiltrate').

The diplomat was accused of being an intruder in the sovereign affairs of the neighboring nation.

Diplomat itu dituduh sebagai penyusup dalam urusan kedaulatan negara tetangga.

Political context.

The architecture was designed to be hostile to any potential intruder, with sharp angles and narrow passages.

Arsitektur itu dirancang untuk memusuhi setiap penyusup potensial, dengan sudut-sudut tajam dan lorong-lorong sempit.

Architectural context.

The intruder's presence catalyzed a series of events that led to the family's disintegration.

Kehadiran penyusup itu mengkatalisasi serangkaian peristiwa yang menyebabkan disintegrasi keluarga.

Complex cause-and-effect structure.

In the realm of quantum physics, the observer is sometimes viewed as an intruder who alters the state of the system.

Dalam ranah fisika kuantum, pengamat terkadang dipandang sebagai penyusup yang mengubah keadaan sistem.

Scientific/Philosophical context.

Common Collocations

masked intruder

unwanted intruder

detect an intruder

armed intruder

potential intruder

silent intruder

repel an intruder

unidentified intruder

digital intruder

domestic intruder

Common Phrases

intruder alert

— A warning that someone has entered a secure area without permission. Often used in sci-fi or security contexts.

The computer voice announced, 'Intruder alert in sector 7!'

feel like an intruder

— To feel unwelcome or out of place in a social situation. It describes a feeling of not belonging.

I felt like an intruder when I walked into their private argument.

ward off intruders

— To prevent people from entering a place without permission. Often involves physical barriers or alarms.

They bought a large dog to ward off intruders.

unwelcome intruder

— A person whose presence is not desired. This can be physical or metaphorical.

The news of the scandal was an unwelcome intruder in his peaceful life.

intruder detection system

— A technical setup (hardware or software) designed to find unauthorized people or data.

The bank installed a state-of-the-art intruder detection system.

catch an intruder

— To find and stop someone who has entered a place illegally.

The night watchman managed to catch an intruder in the warehouse.

scare away an intruder

— To make an intruder leave by making noise, turning on lights, or showing force.

The loud barking of the dog was enough to scare away the intruder.

identify the intruder

— To find out who the person is that entered without permission.

The police used DNA evidence to identify the intruder.

intruder on the property

— A person who is physically present on land they do not own or have permission to be on.

There is an intruder on the property near the north fence.

keep out intruders

— To maintain security so that no unauthorized people can enter.

The main goal of the security team is to keep out intruders.

Often Confused With

intruder vs burglar

A burglar is an intruder who specifically intends to steal. An intruder might not have that intent.

intruder vs stranger

A stranger is just someone you don't know. They are only an intruder if they enter your space without permission.

intruder vs trespasser

A trespasser is a legal term for someone on land without permission. 'Intruder' sounds more active and potentially dangerous.

Idioms & Expressions

"an intruder in the dust"

— A reference to someone who is out of place or unwelcome, often in a historical or Southern Gothic context (referencing William Faulkner).

He felt like an intruder in the dust of his ancestors' old mansion.

literary

"break and enter"

— The act of an intruder forcing their way into a building. While not using the word 'intruder,' it describes the action.

The intruder was charged with break and enter.

legal

"gatecrash a party"

— To go to a party without an invitation, acting as a social intruder.

They decided to gatecrash the wedding reception.

informal

"trespass on someone's time"

— To be an intruder in someone's schedule by taking up their time without permission.

I don't want to trespass on your time, but I have one quick question.

formal

"invade someone's privacy"

— To act as an intruder into someone's personal life or information.

Taking photos of them through the window is a way to invade their privacy.

neutral

"a wolf in sheep's clothing"

— An intruder who pretends to be a friend or a harmless person.

The new employee turned out to be a wolf in sheep's clothing, stealing company secrets.

neutral

"crash the gates"

— To enter a place or event forcefully or without an invitation.

Protesters tried to crash the gates of the embassy.

neutral

"barge in"

— To enter a room or a conversation rudely and without permission, like an intruder.

He just barged in while we were having a private meeting.

informal

"stick one's nose in"

— To be a social intruder by interfering in something that is not one's business.

She's always sticking her nose in other people's problems.

informal

"poke around"

— To look around a place where one might be an intruder, often looking for something.

I found him poking around in my desk drawers.

informal

Easily Confused

intruder vs Invader

Both involve entering without permission.

An invader usually refers to a large group or army entering a country. An intruder is typically an individual entering a building.

The army fought off the invader, while the homeowner called the police about an intruder.

intruder vs Infiltrator

Both enter secretly.

An infiltrator enters to stay for a long time and gather information. An intruder enters and is usually detected or leaves quickly.

The spy was an infiltrator in the government, but the man in the hallway was just an intruder.

intruder vs Interloper

Both are uninvited.

An interloper involves themselves in affairs or social groups where they don't belong. An intruder physically enters a space.

He was an interloper in the family business, but an intruder in their home.

intruder vs Prowler

Both are suspicious people near a house.

A prowler is someone hanging around outside, often looking for a way in. An intruder has already entered.

The prowler was seen in the yard, but the intruder was found in the kitchen.

intruder vs Gatecrasher

Both are uninvited guests.

A gatecrasher is specific to parties and social events. An intruder is a more general and serious term for any uninvited entry.

The gatecrasher just wanted free drinks, but the intruder wanted to steal the safe.

Sentence Patterns

The [animal] barked at the intruder.

The dog barked at the intruder.

There is an intruder in the [place].

There is an intruder in the garden.

The intruder entered through the [opening].

The intruder entered through the back door.

I felt like an intruder at the [event].

I felt like an intruder at the wedding.

The [system] is designed to detect an intruder.

The software is designed to detect an intruder.

The intruder was caught [idiom].

The intruder was caught red-handed.

The intruder exploited a [weakness].

The intruder exploited a gap in the security.

The presence of an intruder catalyzed [consequence].

The presence of an intruder catalyzed a change in policy.

Word Family

Nouns

Verbs

Adjectives

How to Use It

frequency

Common in news, crime fiction, and technical security documents.

Common Mistakes

Using 'intruder' for every stranger. → A stranger is just someone unknown; an intruder is someone uninvited in a private space.
You can meet a stranger at a park, but they are only an intruder if they come into your house without asking.
Saying 'The intruder at the house.' → The intruder in the house.
We use 'in' because the intruder has entered the interior space of the building.
Confusing 'intruder' with 'burglar' in legal contexts. → Use 'intruder' if you don't know if they want to steal; use 'burglar' if they do.
An intruder might just be trespassing. A burglar is specifically there to commit theft.
Pronouncing it as 'IN-tru-der'. → in-TRU-der.
The stress must be on the second syllable for correct English pronunciation.
Using 'intruder' for an annoying guest. → An unwelcome guest or an interloper.
'Intruder' implies a security breach. If you invited them but now want them to leave, they aren't technically an intruder.

Tips

Use 'Intruder' for Security

When writing about home or business safety, 'intruder' is the most professional and clear word to use for someone who shouldn't be there.

Check Your Prepositions

Remember to use 'in' or 'into' with intruder. 'The intruder into the system' or 'The intruder in the house' are correct.

Stress the Middle

Make sure you emphasize the 'tru' sound. It's in-TRU-der. This helps native speakers understand you immediately.

Digital vs. Physical

Don't forget that 'intruder' is very common in IT. If you're studying computer science, this is a key term for security breaches.

Vary Your Synonyms

If you use 'intruder' too many times in a story, switch to 'prowler' or 'uninvited guest' to keep your writing interesting.

Listen for Alarms

In movies and TV shows, the word 'intruder' is almost always followed by an alarm sound or police sirens.

Social Sensitivity

Be careful using 'intruder' for people you know. It's a strong word that implies they are doing something very wrong.

Think of 'Pushing'

The Latin root 'trudere' means to push. Think of an intruder as someone who 'pushes' their way into a place.

Animal Intruders

Use 'intruder' when describing an animal entering another animal's territory. It sounds more scientific and accurate.

CEFR B1 Level

For B1 exams, knowing 'intruder' is important for tasks related to reporting a crime or describing a suspicious event.

Memorize It

Mnemonic

Think of someone who 'IN-TRU-des' (in-throws) themselves into your room. They 'push' (trude) their way 'in'.

Visual Association

Imagine a person wearing a black mask and a striped shirt stepping through a window. This classic 'burglar' image is the easiest way to remember an intruder.

Word Web

Security Alarm Privacy Unwelcome Trespass Hacker Boundary Protection

Challenge

Try to write three sentences using 'intruder': one about a house, one about a computer, and one about a social situation.

Word Origin

The word 'intruder' comes from the Middle English word 'intruden,' which was adapted from the Latin verb 'intrudere.' In Latin, 'in-' means 'into' and 'trudere' means 'to thrust' or 'to push.'

Original meaning: To thrust oneself into a place or a situation without right or welcome.

It belongs to the Indo-European language family, specifically the Italic branch via Latin.

Cultural Context

Be careful when using 'intruder' to describe people in sensitive political situations (like refugees or immigrants), as it can be seen as dehumanizing or biased.

In the UK and US, 'intruder' is the standard term used by police and security companies. It sounds more serious than 'trespasser' but less specific than 'burglar.'

Intruder in the Dust (a novel by William Faulkner) The Intruder (a 2019 psychological thriller film) Intruder (a classic survival horror video game)

Practice in Real Life

Real-World Contexts

Home Security

install an alarm
detect an intruder
scare off a burglar
secure the perimeter

Cybersecurity

unauthorized access
network breach
hacker detection
firewall protection

Nature/Wildlife

territorial dispute
invasive species
defend the nest
unwelcome predator

Social Situations

feel out of place
not invited
private conversation
social boundaries

Legal/Police

trespassing charges
apprehend the suspect
illegal entry
property rights

Conversation Starters

"Have you ever had an intruder in your garden or house?"

"What is the best way to protect a home from an intruder?"

"Do you think a digital intruder is more dangerous than a physical one?"

"Have you ever felt like an intruder at a party where you didn't know anyone?"

"How should the law treat an intruder who enters a house by mistake?"

Journal Prompts

Describe a time when you felt like an intruder in a social situation. Why did you feel that way?

Write a short mystery story that begins with the sound of an intruder in the attic.

If you were designing a high-tech security system to catch an intruder, what features would it have?

Compare the terms 'intruder,' 'guest,' and 'stranger.' How do the meanings change based on the situation?

Discuss the ethical implications of how we treat 'ecological intruders' (invasive species).

Frequently Asked Questions

10 questions

Not necessarily. While the word is often used for criminals like burglars, an intruder can be anyone who enters without permission. This includes someone who is lost, a child wandering into a yard, or a neighbor who enters the wrong apartment by mistake. The defining feature is the lack of permission, not the intent to commit a crime.

The main difference is the context and connotation. 'Trespasser' is a legal term used for someone who enters land or property without permission. 'Intruder' is a more general term that often implies a more threatening or active presence, especially inside a building. You might see a 'No Trespassing' sign, but you would call the police about an 'intruder' in your house.

Yes, animals can be described as intruders. If a bear enters a campsite or a strange dog enters your living room, they are intruders. In nature documentaries, animals that enter the territory of another animal are frequently called intruders. It highlights the violation of a space that 'belongs' to someone else.

In cybersecurity, an intruder is a person who gains unauthorized access to a computer system or network. You can say, 'The firewall blocked the intruder' or 'We are investigating a digital intruder.' It is often used in the phrase 'Intruder Detection System' (IDS), which is software that monitors for security breaches.

It is a neutral to formal word. It is appropriate for news reports, police statements, and technical writing. In very casual conversation, people might use 'random person' or 'guy who broke in,' but 'intruder' is widely understood and used in everyday English.

Common adjectives include 'masked,' 'armed,' 'silent,' 'unwanted,' 'potential,' 'unidentified,' and 'persistent.' These words help describe the appearance, threat level, or behavior of the person who has entered without permission.

You can, but 'gatecrasher' is a more specific and common term for someone who attends a party without an invitation. Using 'intruder' for a party guest makes the situation sound much more serious and potentially dangerous.

The verb form is 'intrude.' For example, 'I don't want to intrude on your conversation.' The noun 'intrusion' refers to the act itself, while 'intruder' refers to the person who does it.

Yes, 'intruder' is a countable noun. You can say 'one intruder,' 'two intruders,' or 'many intruders.' The plural form is created by adding an 's' at the end.

This is a metaphorical expression. It means to feel like you don't belong in a certain place or social group, or that your presence is making others uncomfortable. For example, 'I felt like an intruder at their family dinner because they were all arguing.'