noun An object designed to open and close a lock or to activate or deactivate something, especially a length of metal inserted into a narrow opening on the lock to which it is matched (keyed).
This key is to the backdoor.
noun A card or fob that electronically signals a lock to open.
My hotel room key is a card.
noun A piece of information (e.g., a password or passphrase) used to encode or decode a message or messages.
noun A password restricting access to an IRC channel.
noun A key bump (A portion of a recreational drug in powdered form that is snorted from a key or similar small object.)
I saw him snort a key at the party.
noun Any of various tools comparable to a wrench (spanner) or driver, which have a specialized end that matches a fixture, analogously to a lock's key.
The exterminator will bring a key thingy that can open the bait boxes.
noun An object designed to fit between two other objects (such as a shaft and a wheel) in a mechanism and maintain their relative orientation.