Miter

A miter is an angled cut, most commonly 45 degrees, made so two pieces meet at a corner (typically 90 degrees total) without either piece's end grain showing -- the standard joint behind picture frames, box corners, and molding meeting at a wall corner. Because a plain miter joint is entirely end-grain-to-end-grain, it glues relatively poorly on its own (the same problem covered in Butt Joint's discussion of end grain), which is why miters are frequently reinforced with splines, biscuits, or corner keys rather than relying on glue alone.

A compound miter combines an angled cut with a simultaneous bevel (blade tilt), needed for crown molding and any frame that isn't meeting at a flat, square corner -- considerably fussier to calculate and set up than a simple flat 45-degree miter.