here's another def, I'm looking for a picture...
Header-
(a) A beam placed perpendicular to joists and to which joists are nailed inframing for a chimney, stairway, or other opening.
(b) A wood lintel.
(c) The horizontal structural member over an opening (for example over a door or window).

here we go:
http://www.claimrep.com/Pictures/Header2256.jpg


formerly known as etaoin...