Definition of "soffit"

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.

2014 September 11, Jeff Howell, “Home improvements: The dangers of abestos roofs ”, in The Daily Telegraph (Property)‎