The Collaborative International Dictionary of English v.0.48:

Mortise \Mor"tise\, n. [F. mortaise; cf. Sp. mortaja, Ar.
   murtazz fixed, or W. mortais, Ir. mortis, moirtis, Gael.
   moirteis.]
   A cavity cut into a piece of timber, or other material, to
   receive something (as the end of another piece) made to fit
   it, and called a tenon.
   [1913 Webster]

   Mortise and tenon (Carp.), made with a mortise and tenon;
      joined or united by means of a mortise and tenon; -- used
      adjectively.

   Mortise joint, a joint made by a mortise and tenon.

   Mortise lock. See under Lock.

   Mortise wheel, a cast-iron wheel, with wooden clogs
      inserted in mortises on its face or edge; -- also called
      mortise gear, and core gear.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mortise joint
    n 1: a gliding joint between the distal ends of the tibia and
         fibula and the proximal end of the talus [syn: ankle,
         ankle joint, mortise joint, articulatio talocruralis]
    2: a joint made by inserting tenon on one piece into mortise
       holes in the other [syn: mortise joint, mortise-and-tenon
       joint]