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: Webster's Revised Unabridged Dictionary (1913)