A soffit is a section of ceiling that is dropped well below the underside of the roof or floor above. What it creates is an enclosed space above the ceiling, often filled with heating and cooling ducts. A soffit is most often designed for an architectural affect – the dropping of the ceiling helping to create an area of space differentiated from other areas. Soffits can also be exterior and the term is also employed to describe the ceiling-like underside of extended eaves (known more obscurely as the “plantia”).
The term owes is origins to a Latin verb meaning to fix underneath, and it is the “underneath” part that we have carried into current architectural parlance.