Also: hypocaustum, rarely hypocausis

The heating system used in Roman-style bathhouses to heat rooms and pools. It consisted of a series of short columns/pillars (pilae) supporting the circulation floor (suspensura). The hollow space underneath the walking-level was heated by one or more furnaces. The hot gases of these furnaces heated the floor and hence the room above. The term is a latinised form of the Greek ὑπόκαυστον, meaning “heated from below”.

L: Remains of the hypocaust in the baths of Ferentum, in Lazio (photo Allison Smith).
R: The hypocaust in the caldarium of the South Gate Baths in Perge, Turkey. The suspensura rests on brick arches rather than on the more common square pilae (photo Konogan Beaufay).