If a building — or sometimes furniture, a bridge, a school block — is prefabricated, the parts are made first, in a factory, in standard sizes, then taken to the place and fitted together. You do not build the whole thing from scratch on the land. Walls may arrive on a truck; rooms may already be made. This is the full, slightly technical word; prefab is the short, informal one. The idea is always “made in pieces beforehand, assembled where you need it.” People use it most for buildings.