Solid hardwood flooring is milled from a single piece of solid wood. As such, the boards can withstand heavy foot traffic and be refinished almost limitlessly. However, they are prone to expanding and contracting with changes in relative humidity.
Engineered hardwood flooring is a manufactured product with multiple cross layers of plywood and a thin veneer of real wood on top. This construction counteracts the tendency of wood to buckle or crack, creating more durable flooring that can be installed anywhere, including the basement and bathrooms. However, the thin veneer of engineered wood floors limits refinishing to just once or twice at most.