soffit

Etymology

From French soffite, from Italian soffitto, from sof- (“under”) + past participle of figgere (“to fix”).

noun

1. (architecture) The visible underside of an arch, balcony, beam, cornice, staircase, vault or any other architectural element.
   If the soffit is to be sloping, simply attach the soffit board(s) to the underside of the rafters, butting the edges tight against the fascia board in front and the barn siding in the back. 1983, Monte Burch, Building Small Barns, Sheds & Shelters, page 110
   A soffit is the underside of any part of a building's structure. Most people know about the soffit boards below their fascia boards (which cap off the ends of a roof's rafters). But if you walk through a brick-built tunnel and glance upwards, for example, you will be looking at the soffit of the arch. 11 September 2014, Jeff Howell, “Home improvements: The dangers of abestos roofs [print version: Where can I find help – at the double?, 6 September 2014, p. P13]”, in The Daily Telegraph (Property)
2. (pipe technology) The top point of the inside open section of a pipe or box conduit.
   The elevation of the pipe soffit is 10.4 meters above mean sea level.