Solid Hardwood
Hardwood flooring milled from a single piece of wood. Nail-down install over plywood subfloor only.
Solid hardwood is just what it sounds like: a single 3/4"-thick piece of milled hardwood (oak, maple, walnut, hickory, etc.). It installs by nail-down over a plywood subfloor and can be sanded and refinished many times over its 50-100 year service life.
Solid hardwood is dimensionally less stable than engineered. It can't go below grade (basement), can't go over concrete slab, and can't go with radiant heat. It needs a strict 30-50% relative humidity environment year-round.
Pricing runs $5-$15 per square foot for material, $4-$8 per square foot for install. The premium over engineered is justified by the longer service life and refinish potential, but the install constraints are real.