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 pilae made of square bessales (photo Konogan Beaufay).