key factor

Derived from the combination of 'key' (meaning essential, crucial, or vitally important, dating back to the late 14th century, referencing the instrument used to open locks) and 'factor' (meaning a circumstance, fact, or influence tending to bring about a result, from the Latin 'factor' meaning 'doer' or 'maker'). The combination emerged to specifically denote an element that holds significant importance in determining an outcome, much like a key is essential to unlocking something.