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