a hole or socket cut into a piece of wood or stone to receive a matching part; a mortise (often in the idiom 方枘圆凿 fāng​ruì​yuán​záo)

(bound form) tenon; the protruding part of a joint in woodwork or metalwork that fits into a hole. Also used figuratively in phrases like 方枘圆凿 fāng​ruì​yuán​záo to describe things that are incompatible or do not fit together