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From The Collaborative International Dictionary of English v.0.48:
Key \Key\ (k[=e]), n. [OE. keye, key, kay, AS. c[ae]g.] 1. An instrument by means of which the bolt of a lock is shot or drawn; usually, a removable metal instrument fitted to the mechanism of a particular lock and operated by turning in its place. [1913 Webster] 2. A small device which is inserted into a mechanism and turned like a key to fasten, adjust, or wind it; as, a watch key; a bed key; the winding key for a clock, etc. [1913 Webster] 3. One of a set of small movable parts on an instrument or machine which, by being depressed, serves as the means of operating it; the complete set of keys is usually called the keyboard; as, the keys of a piano, an organ, an accordion, a computer keyboard, or of a typewriter. The keys may operate parts of the instrument by a mechanical action, as on a piano, or by closing an electrical circuit, as on a computer keyboard. See also senses 12 and 13. [1913 Webster +PJC] 4. A position or condition which affords entrance, control, pr possession, etc.; as, the key of a line of defense; the key of a country; the key of a political situation. Hence, that which serves to unlock, open, discover, or solve something unknown or difficult; as, the key to a riddle; the key to a problem. Similarly, see also senses 14 and 15. [1913 Webster] Those who are accustomed to reason have got the true key of books. --Locke. [1913 Webster] Who keeps the keys of all the creeds. --Tennyson. [1913 Webster] 5. That part of a mechanism which serves to lock up, make fast, or adjust to position. [1913 Webster] 6. (Arch.) (a) A piece of wood used as a wedge. (b) The last board of a floor when laid down. [1913 Webster] 7. (Masonry) (a) A keystone. (b) That part of the plastering which is forced through between the laths and holds the rest in place. [1913 Webster] 8. (Mach.) (a) A wedge to unite two or more pieces, or adjust their relative position; a cotter; a forelock. See Illusts. of Cotter, and Gib. (b) A bar, pin or wedge, to secure a crank, pulley, coupling, etc., upon a shaft, and prevent relative turning; sometimes holding by friction alone, but more frequently by its resistance to shearing, being usually embedded partly in the shaft and partly in the crank, pulley, etc. [1913 Webster] 9. (Bot.) An indehiscent, one-seeded fruit furnished with a wing, as the fruit of the ash and maple; a samara; -- called also key fruit. [1913 Webster] 10. (Mus.) (a) A family of tones whose regular members are called diatonic tones, and named key tone (or tonic) or one (or eight), mediant or three, dominant or five, subdominant or four, submediant or six, supertonic or two, and subtonic or seven. Chromatic tones are temporary members of a key, under such names as " sharp four," "flat seven," etc. Scales and tunes of every variety are made from the tones of a key. (b) The fundamental tone of a movement to which its modulations are referred, and with which it generally begins and ends; keynote. [1913 Webster] Both warbling of one song, both in one key. --Shak. [1913 Webster] 11. Fig: The general pitch or tone of a sentence or utterance. [1913 Webster] You fall at once into a lower key. --Cowper. [1913 Webster] 12. (Teleg.) A metallic lever by which the circuit of the sending or transmitting part of a station equipment may be easily and rapidly opened and closed; as, a telegraph key. [Webster 1913 Suppl.] 13. any device for closing or opening an electric circuit, especially as part of a keyboard, as that used at a computer terminal or teletype terminal. [PJC] 14. A simplified version or analysis which accompanies something as a clue to its explanation, a book or table containing the solutions to problems, ciphers, allegories, or the like; or (Biol.) a table or synopsis of conspicuous distinguishing characters of members of a taxonomic group. [Webster 1913 Suppl.] 15. (Computers) A word or other combination of symbols which serves as an index identifying and pointing to a particular record, file, or location which can be retrieved and displayed by a computer program; as, a database using multi-word keys. When the key is a word, it is also called a keyword. [PJC] Key bed. Same as Key seat. Key bolt, a bolt which has a mortise near the end, and is secured by a cotter or wedge instead of a nut. Key bugle. See Kent bugle. Key of a position or Key of a country. (Mil.) See Key, 4. Key seat (Mach.), a bed or groove to receive a key which prevents one part from turning on the other. Key way, a channel for a key, in the hole of a piece which is keyed to a shaft; an internal key seat; -- called also key seat. Key wrench (Mach.), an adjustable wrench in which the movable jaw is made fast by a key. Power of the keys (Eccl.), the authority claimed by the ministry in some Christian churches to administer the discipline of the church, and to grant or withhold its privileges; -- so called from the declaration of Christ, "I will give unto thee the keys of the kingdom of heaven." --Matt. xvi. 19. [1913 Webster]