The part of the side of a window or door opening that is between the outer surface of a wall and the window or door frame. b. The whole side of such an opening; the jamb. 2. The framework of a motor vehicle window.
or, in the (perhaps clearer) words of the OED,
A side of an opening or recess which is at right angles to the face of the work; esp. the vertical side of a doorway or window-opening between the door- or window-frame and the arris ['the sharp edge formed by the angular contact of two plane or curved surfaces'].
That’s interesting enough, but what’s amazing is that it has nothing to do with the verb reveal (which is related to veil); it’s from a totally different (and obsolete) verb revale ‘to lower, bring down,’ which is related to vale and valley. As Mark says, Live and learn.