The Collaborative International Dictionary of English v.0.48:

Keyhole \Key"hole`\, n.
   1. A hole or apertupe in a door or lock, for receiving a key.
      [1913 Webster]

   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.
          [1913 Webster]

   Keyhole limpet (Zool.), 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 (Zool.), 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.
      [1913 Webster]