Definition from Merriam-Webster:
1. noun
- a usually metal instrument by which the bolt of a lock is turned
- a device having the form or function of a key
- a means of gaining or preventing entrance, possession, or control
- an instrumental or deciding factor
- something that gives an explanation or solution
- a list of words or phrases giving an explanation of symbols or abbreviations
- a series of phrases or statements that describe observable characteristics by which plants or animals of a particular group differ and are used to aid in identification