Haunch

A haunch is a short, stepped-down extension left on part of a tenon's shoulder, most commonly where a frame rail's mortise-and-tenon joint runs into a groove cut for a floating panel. Without the haunch, the panel groove would run all the way through the mortise, leaving a visible gap at the outside edge of the frame once assembled; the haunch fills that gap while still letting the main tenon do its structural work.

It's a small detail that's easy to overlook in layout but genuinely matters -- a haunched mortise-and-tenon joint is standard on any frame-and-panel door or frame where the panel groove and the joint occupy the same edge.