Polyurethane is a synthetic resin used in some modern finishes.

Polyurethane (commonly shortened to poly) is a synthetic resin that is used to make a few different types of modern finish. Some primarily consist of polyurethane while in others, including all the common polyurethane varnishes (both oil-based and waterbased), the polyurethane is an additive and may be present in only small amounts. It is added to improve wear and scratch resistance.