caldarium
nounUK /kælˈdɛəɹi.əm/ · US /kælˈdɛɹi.əm/
Etymology Unadapted borrowing from Latin caldārium (“room containing warm water for bathing”), substantive of caldārius (“of, pertaining to or suitable for bathing”), from caldus (“warm, hot”) + -ārius, alternative form of calidus, from caleō (“to be warm or hot; glow”).
1
Historical In Roman baths, the hottest room, with a plunge-pool. It preceded the tepidarium and frigidarium.
2
In modern spas, a room with a hot floor.