Definition of key:
an object designed to open and close a lock
; 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 solving this problem..."
"he is the key player for his soccer team."
to fit (a lock) with a key; to fit (pieces of a mechanical assembly) with a key to maintain the orientation between them
"our instructor told us to key in our user IDs."