Definition: Subfloor refers to any kind of non-decorative, usually structurally-superior flooring (usually plywood or cement board like Wonderboard) underneath a decorative floor.
Decorative floors, such as ceramic tile, solid wood, laminate, engineered wood, and others, all employ sub-floor in their construction. In most cases, subfloor is the only layer between the decorative flooring and the joists, and is absolutely necessary for the installation of the decorative layer.
Without the subfloor, the decorative layer would have nothing to rest on. In other cases, there may already be an existing floor, but it is not of a high enough quality for the decorative flooring to rest on. For instance, old and warped hardwood flooring will often get a thin layer of subfloor before the decorative layer is laid down.
Decorative floors, such as ceramic tile, solid wood, laminate, engineered wood, and others, all employ sub-floor in their construction. In most cases, subfloor is the only layer between the decorative flooring and the joists, and is absolutely necessary for the installation of the decorative layer.
Without the subfloor, the decorative layer would have nothing to rest on. In other cases, there may already be an existing floor, but it is not of a high enough quality for the decorative flooring to rest on. For instance, old and warped hardwood flooring will often get a thin layer of subfloor before the decorative layer is laid down.
Alternate Spellings: sub floor, sub-floor, subflooring, sub flooring
Examples:
The floor installers were forced to put down a layer of quarter-inch plywood subfloor on the cracked hardwood, before putting down the Pergo.

