key

The word “key” means a shaped piece of metal used to open or lock a particular lock. In plain terms, most often either the metal object that opens a lock, or (as an adjective) something essential and central to a situation.

“Key” is pronounced kee (/ki/ in IPA).

Here is “key” used in a sentence: “She forgot her house key at the office again.”

Depending on how it is used, “key” can be a noun and an adjective.

The word “key” comes from Old English. From Old English cæg, of uncertain deeper origin, though it may ultimately trace back to a root meaning "stake" or "peg" — early keys were simple rods used to lift a bar across a door.

“Key” is a very common word in modern English, usually learned at around the A1 level.

“Key” has 7 distinct senses listed in this dictionary. The most common is: a shaped piece of metal used to open or lock a particular lock.

A lock is the fixed mechanism that secures a door, drawer, or gate, while a key is the small shaped piece of metal that operates it. The two are made as a matched pair: the key's unique cuts fit the internal pins of one specific lock, so only that key can open it.

Common types include house keys and car keys for everyday access, master keys that open several different locks in a building, and skeleton keys designed to work with many older locks. There are also padlock keys and safe keys. Each is cut to match the particular lock it is meant to operate.

A familiar example is the metal house key you use to unlock your front door. When you slide it in and turn, its jagged edge lifts the pins inside the lock to exactly the right height, allowing the cylinder to rotate and the door to open. A car ignition key works on the same principle.