Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.) An indehiscent, one-seeded fruit furnished with a
      wing, as the fruit of the ash and maple; a samara; --
      called also {key fruit}.

   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.

                 Both warbling of one song, both in one key.
                                                  --Shak.

   11. Fig: The general pitch or tone of a sentence or
       utterance.

             You fall at once into a lower key.   --Cowper.

   {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 {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.