soffit

The underside of a structural component, such as a beam, arch, staircase, or cornice.

In architecture: The under horizontal face of an architrave between columns

The lower surface of an arch.

The ceiling of a room, when divided by cross-beams into panels, compartments, or lacunaria.

The under face of an overhanging cornice, of a projecting balcony, an entablature, a staircase, etc.

In scenepainting, a border. See scene, 4.

The under side of the subordinate parts and members of buildings, such as staircases, entablatures, archways, cornices, or the like. See Illust. of lintel.

The visible underside of an arch, balcony, beam, cornice, staircase, vault or any other architectural element.

The top point of the inside open section of a pipe or box conduit.

the underside of a part of a building (such as an arch or overhang or beam etc.)