​MORTISE, or Mortice (adapted from the Fr. mortaise; cf. Ital. mortise and Spanish mortaja; the origin is unknown; Celtic equivalents, such as Gaelic moirteis, are of French origin), a term for a socket or cavity cut in a piece of wood, or other material, into which a corresponding projecting end, a “ tenon,” fits, the two when fitted together forming a “ mortise-joint,” for fastening two beams or other pieces of timber together.