The term flooring refers to the lower enclosed surface of indoor spaces. This may be part of the floor structure, such as the top surface of a concrete slab or floorboards, but it is usually a permanent covering placed on the floor. We also use flooring to describe the process of laying flooring materials.
The floor under the floor is called the floor covering that supports the floor. Special substructures such as floating floors or sprung floors may be placed on top of another subfloor to provide structural strength. Underground floors (basements) or ground floors in buildings without basements usually have a concrete foundation. Bottom floors above ground (on the floor) usually have a plywood underlayer.