key

noun

1. 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).
2. A card or fob that electronically signals a lock to open.
3. A piece of information (e.g., a password or passphrase) used to encode or decode a message or messages.
4. A password restricting access to an IRC channel.
5. A key bump (A portion of a recreational drug in powdered form that is snorted from a key or similar small object.)
6. 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.
7. A cotter pin.
8. A piece of wood used as a wedge.
9. The last board of a floor when laid down.
10. A keystone.
11. That part of the plastering which is forced through between the laths and that holds the rest in place.
12. A wooden wedge, driven sideways between a bullhead rail and a cast-iron chair, to keep the rail securely in position.
13. A crucial step or requirement.
14. An idea, theory or observation that explains, justifies or renders (something) comprehensible.
15. A deciding factor.
16. A small guide explaining symbols, abbreviations or terminology, especially the legend on a map or chart.
17. A guide to the correct answers of a worksheet or test.
18. A series of logically organized groups of discriminating information which aims to allow the user to correctly identify a taxon; a taxonomic key.
19. A value that uniquely identifies an entry in a container.
20. In a relational database, a field used as an index into another table (not necessarily unique).