Real Dictionary

key
a metal instrument, usually of a specifically contoured shape, that is made to fit a lock and, when rotated, operates the lock's mechanism [...]
an element or cause that contributes to a result [...]
the act or an instance of transcribing or the state of being transcribed [...]
The key person or thing in a group is the most important one.
The keys of a piano or organ are the long narrow pieces of wood or plastic that you press in order to play it.
The keys on a computer keyboard or typewriter are the buttons that you press in order to operate it.
countable noun: A key is a specially shaped piece of metal that you place in a lock and turn in order to open or lock a door, or to start or stop the engine of a vehicle.
countable noun: The key on a map or diagram or in a technical book is a list of the symbols or abbreviations used and their meanings.
variable noun: In music, a key is a scale of musical notes that starts on one specific note.