What is the difference between solid and engineered hardwood?

Solid hardwood is one piece of wood through its full thickness. Engineered hardwood is a real wood wear layer bonded to a plywood or composite core. The practical difference is where each can go. Solid wants a wood subfloor and reacts strongly to moisture and humidity, which rules it out over most concrete. Engineered is dimensionally more stable, can be glued or floated over a slab, and is therefore the usual answer in a condo or a basement. Cost is close: $8 to $16 per square foot installed for engineered against $9 to $18 for solid.