Source : Webster's Revised Unabridged Dictionary (1913)

Keyhole \Key"hole`\, n.
   1. A hole or apertupe in a door or lock, for receiving a key.

   2.
      (a) (Carp.) A hole or excavation in beams intended to be
          joined together, to receive the key which fastens
          them.
      (b) (Mach.) a mortise for a key or cotter.

   {Keyhole limpet} (Zo["o]l.), a marine gastropod of the genus
      Fissurella and allied genera. See {Fissurella}.

   {Keyhole saw}, a narrow, slender saw, used in cutting
      keyholes, etc., as in doors; a kind of compass saw or fret
      saw.

   {Keyhole urchin} (Zo["o]l.), any one of numerous clypeastroid
      sea urchins, of the genera {Melitta}, {Rotula}, and
      {Encope}; -- so called because they have one or more
      perforations resembling keyholes.

Source : WordNet®

keyhole saw
     n : a handsaw for cutting short radius curves; similar to a
         compass saw