In architecture, an open space below a floor to allow the passage of hot air and smoke in order to heat the room above.

This type of heating was developed to a high degree by the Romans who used it not only in the warm and hot rooms of the baths, but also almost universally in private houses in the northern provinces.

Many examples of such hypocausts exist in villa and house foundations in Roman centers in Germany and England. The usual custom was to lead the smoke from a hypocaust into a single vertical flue through which it escaped into the open air.

The word comes from Latin hypocaustum, and previously from Greek hypokauston and ultimately from Greek hypokaiein, "to light a fire beneath".