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
      [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
      [1913 Webster]

            Those who are accustomed to reason have got the true
            key of books.                         --Locke.
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            Who keeps the keys of all the creeds. --Tennyson.
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   5. That part of a mechanism which serves to lock up, make
      fast, or adjust to position.
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   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.
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   11. Fig: The general pitch or tone of a sentence or
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             You fall at once into a lower key.   --Cowper.
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   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
       [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.

   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.

   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,

   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]

From The Collaborative International Dictionary of English v.0.48:

keyword \key"word`\, n.
   A word used as an entry point into an index which serves to
   identify files, records, texts, or other data containing the
   keyword or some related word, such as a synonym. It is a type
   of key[15]; as, a boolean combination of keywords is more
   effective for information retrieval than a single key[15].
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