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 their relative orientation.

A crucial step or requirement.

"The key to solving this problem is persistence."

To fit (a lock) with a key.

To fit (pieces of a mechanical assembly) with a key to maintain the orientation between them.

To mark or indicate with a symbol indicating membership in a class.

Indispensable, supremely important.

"He is the key player on his soccer team."

Important, salient.

"She makes several key points."