Window Reveal
Originally the part of the window or door surround revealed by setting back the architrave. Colloquially, the reveal is extended to include the decorative surround or framing at the sides and top of the inside of a window or door. The reveal is often timber but can also be stone, brick, or plasterboard.
Another way of describing the reveal is the side of an opening for a window, doorway, or the like, between the door frame or window frame and the outer surface of the wall; or, where the opening is not filled with a door, etc., the whole thickness of the wall; the jamb.