Model + intelligent 3D geometry (the base layer). BIM "dimensions" describe what layer of information is added on top of the 3D model. Think of 3D as the base - each additional dimension enriches the model with a new type of data.

The base model. Walls, floors, roofs, columns, ducts, pipes - all modeled as intelligent 3D objects in space. This is what most people think of when they hear "Revit model."