A precise recess cut into a timber or board, designed to house a tenon, lock body, or hinge leaf. In structural joinery, it is usually a deep, rectangular hole with flat interior walls and square corners. The structural integrity of the resulting joint relies entirely on the tight friction fit between the mortise walls and the inserted piece.
Clean out the bottom of that mortise with a chisel so the tenon seats all the way to the shoulder.
Related terms
Mortise-and-tenon joint
A traditional wood joint connecting two pieces at a right angle by inserting a peg into a hole.
Mortiser
A specialized woodworking machine designed to cut square or rectangular holes.
Tenoned
Machined or cut to feature a projecting tongue meant to fit into a mortise.
Dovetail
A flaring, wedge-shaped interlocking joint that resists being pulled apart.