(adj.) 1. Pertaining to an installation prep that typically leaves the bolt face plate and a perpendicular surface of the lock visible, with visible surfaces being flush mounted.
(v.) 2. To make a cavity in a door or drawer that is open on the edge and on one side of the door or drawer.
(adj.) 3. Pertaining to a locking device designed to fit in a half mortise prep.

Source: Lock Industry Standards and Training (LIST) Council