The word thermae is a Latin borrowing from the Greek adjective thermos, therme, thermon (hot).
c.f.Thermopylae (the hot gates, gates of fire) thermae sc. aquae means "hot waters, hot springs".
Building layout
Within the building the baths were divided according to gender. Each gender had three pools: a hot one, a lukewarm one and a cool one. They were respectively called:
the caldarium (L. cal(i)dus, -a,-um "hot" cf. calor orig, calos, caloris m)
the tepidarium (L. tepidus,-a,-um "lukewarm" cf. L. tepeo)
the frigidarium (Latin frigidus,-a,-um "cold")
sometimes there was also a steam bath: the sudatorium
The baths often included, aside from the three main rooms, listed above, a palaestra, or outdoor gymnasium where men would engage in various ball games and exercises. There, inter alia, weights were lifted and the discus thrown. Men would oil themselves and removes the excess with a strigil (c.f. the well known Apoxyomenus of Lysippus from the Vatican Museum).
The changing room was known as the apotyterium (Greek apotyterion, apo + duo "to take off" here of clothing).
Location
Baths sprung up all over the empire. Where natural hot springs existed (as in Bath, England) thermae were built around them. Alternatively a system of hypocausta (Greek hypocauston < hypo "below" + kaio "to burn") were utilized to heat the waters.
The RomanBaths are a tourist attraction and historical place of interest in the English city of Bath.
The Baths of Caracalla, the second largest baths complex in ancient Rome, were built between 212 and 219 A.D. by the emperor Marcus Aurelius Antoninus, better known by his nickname Caracalla.
The baths were fed by a branch of the Aqua Marcia aqueduct, which brought pure water to Rome from springs in the hills near Subiaco, over 90 km away.
In ancient Rome, wealthy citizens would have had bathing facilities in their own homes, but there were also public baths; balnea were built and run by individuals for their own profit, and thermae, great baths built for the public by wealthy citizens, or by emperors, were either free or extremely cheap.
Roman public baths were the centre of social life and a place for relaxation and recreation and visits would involve other activities, such as sport, exercise, or massage.
Bathing, especially in chilly northern Europe, came to be regarded as unhealthy and was frowned upon as an indulgence.