Subfloor

Underlying flooring that supports a finished floor and can be used as a structural support for lateral loads. 

Example: The floors in your home consist of several layers. The joists are at the bottom, with the subfloor and underlayment in the middle, and the surface flooring on top. In a basement, the subfloor may sit directly on the concrete slab, with a moisture barrier between the two. In the event of water damage, it’s important that the subfloor be dried as soon as possible.