Fabulous! That's a word I use ALL THE TIME in my work -- because the reveals at the entrance to most sixth-century Chinese cave temples are elaborately carved with guardian figures (see this site, about one of the caves I'm working on, where they are sadly referred to as "jambs" rather than reveals). But I never knew the etymology.

Ditto about the etymology and about using the word all the time - in translating guides to churches and castles in Central Europe.

And I've been to Longmen, but it was in 1975. I remember it well!

I couldn't find any official reference but in my everyday specs- and notes-to-millworkers- writing on architectural drawings I use reveal all the time.
It means deliberate recess in millwork detail, not necessarily doorframes or doors, although could be applied there too. Could be specified for vertical wood panels, in different width and depth, on furniture (just yesterday I finished detail for custom console table with 1/4" ebonized reveal on perimeter) or baseboards.

And for us lowbrows, the reveal is that portion of the cable show Trading Spaces when the homeowners see how their neighbors have remodeled their room.

I recall a discussion on "The Late Show with David Letterman" about a billbord that displayed the "reveal" of a woman's breast; the reveal basically being the side of the breast.