Thermae

In ancient Rome, thermae (from Greek θερμός thermos, "hot") and balneae (from Greek βαλανεῖον balaneion) were facilities for bathing. Thermae usually refers to the large imperial bath complexes, while balneae were smaller-scale facilities, public or private, that existed in great numbers throughout Rome.[1]

Roman public baths in Bath, England. The entire structure above the level of the pillar bases is a later reconstruction.
Thermae Maiores, Aquincum, Budapest
The mosaics of the thermal baths
The thermal baths from inside

Most Roman cities had at least one – if not many – such buildings, which were centres not only for bathing, but socializing and reading as well. Bath-houses were also provided for wealthy private villas, town houses, and forts. They were supplied with water from an adjacent river or stream, or within cities by aqueduct. The water would be heated by fire then channelled into the caldarium (hot bathing room). The design of baths is discussed by Vitruvius in De architectura (V.10).