红绿灯。

You use it just like 'traffic light' in English. You can 'wait' for it, 'cross' it, or 'look' at it. It usually acts as a landmark when giving directions. You might say 'turn left at the 红绿灯.' It is a physical object, not a metaphor for stopping or going.

Use it whenever you are navigating a city. It is perfect for talking to taxi drivers. Use it when teaching a child how to cross the street safely. It is helpful when meeting friends at a specific corner. You will hear it constantly in GPS navigation apps.

Don't use it to describe a party light or a flashlight. It only refers to the signals on the road. Avoid using it to describe someone's 'stop and go' personality. Chinese has other metaphors for that. It is a literal term, not a poetic one. Don't use it for a 'green light' meaning 'permission' either.

In China, traffic lights are the heartbeat of the chaotic city flow. Pedestrians often wait in large groups before crossing together. This is sometimes jokingly called 'Chinese-style crossing.' However, modern cities are very strict about 红绿灯 rules now. Cameras are everywhere to catch those who 'run the red light.' Interestingly, red is a lucky color in China. But at a 红绿灯, red still means 'stop' just like everywhere else.

The formal name is 交通信号灯 (traffic signal light). You will rarely hear people say that in conversation. If you want to say 'run a red light,' use 闯红灯. To say 'the light turned green,' say 绿灯亮了. It is a very stable and predictable phrase across all of China.