As a noun, a key is a small piece of metal used to lock or unlock something, or a set of answers or explanations. As an adjective, key means very important or essential.
From Old English “cǣg” meaning “key”, of uncertain origin. The idea of something that opens or explains led to figurative uses like an “answer key” or “key idea”.
The physical key, which opens doors, inspired a mental key, which opens understanding. That’s why we talk about “key facts” or “key skills”—they unlock everything else. The word quietly turns learning into a series of doors and locks.
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