1. noun /kiː/
a) An object designed to open and close a lock.
the key to solving this problem...
b) An object designed to fit between two other objects (such as a shaft and a wheel) in a mechanism and maintain the orientation between them.
the key to winning this game
2. adjective /kiː/
a) Indispensable.
He is the key player for his soccer team.
b) Important, salient.
He is the key witness.
3. verb /kiː/
a) To fit (a lock) with a key.
Our instructor told us to key in our user IDs.
b) To fit (pieces of a mechanical assembly) with a key to maintain the orientation between them.
He keyed the car that had taken his parking spot.
4. adverb /kiː/
Wikipedia foundation.