Subfloor
The structural floor surface — plywood, OSB, or concrete slab — that supports the underlayment and finish flooring.
The subfloor is the load-bearing layer beneath your finish flooring. In residential construction, it's typically plywood or OSB on a wood joist system. In slab-on-grade construction, the subfloor IS the concrete slab.
Subfloor condition determines install method. A flat, dry, level subfloor takes any flooring product. A subfloor with high moisture, deflection, or out-of-flat conditions needs prep — often self-leveling compound, sister joists, or a moisture mitigation membrane — before any finish flooring goes down.
Document subfloor condition with photos and moisture readings before install starts. This is the single best protection against the customer call that says "you guys did something wrong" six months later.