KEY

noun

1. An object designed to open and close a lock.
2. An object designed to fit between two other objects (such as a shaft and a wheel) in a mechanism and maintain their relative orientation.
3. A crucial step or requirement.

verb

1. To fit (a lock) with a key.
2. To fit (pieces of a mechanical assembly) with a key to maintain the orientation between them.
3. To mark or indicate with a symbol indicating membership in a class.

adjective

1. Indispensable, supremely important.
2. Important, salient.