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]

The Collaborative International Dictionary of English v.0.48:

Fissurella \Fis`su*rel"la\, n. [NL., dim. of L. fissura a
   fissure.] (Zool.)
   A genus of marine gastropod mollusks, having a conical or
   limpetlike shell, with an opening at the apex; -- called also
   keyhole limpet.
   [1913 Webster]

WordNet (r) 3.0 (2006):

keyhole limpet
    n 1: marine limpet having a conical shell with an opening at the
         apex [syn: keyhole limpet, Fissurella apertura,
         Diodora apertura]