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.
2. An instrument which is turned like a key in fastening or adjusting any mechanism; as, a watch key; a bed key, etc.
3. That part of an instrument or machine which serves as the means of operating it; as, a telegraph key; the keys of a pianoforte, or of a typewriter.
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. Those who are accustomed to reason have got the true key of books. Locke. Who keeps the keys of all the creeds. Tennyson.
5. That part of a mechanism which serves to lock up, make fast, or adjust to position.
6. (Arch.) (a) A piece of wood used as a wedge. (b) The last board of a floor when laid down.
7. (Masonry) (a) A keystone. (b) That part of the plastering which is forced through between the laths and holds the rest in place.
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.
9. (Bot.)
Defn: An indehiscent, one-seeded fruit furnished with a wing, as the fruit of the ash and maple; a samara; — called also key fruit.