A caldarium was a room with a hot bath, used in a Roman bath complex.
This was a very hot and steamy room heated by a hypocaust an underground celebration system. This was the hottest room in the regular bathroom sequence: after sweating in the caldarium, users were able to access the tepidarium and frigidarium
In the caldarium there would be a hot water bath (alveus, warm pool or solium) sunk into the ground, sometimes there was even a laconicum, a hot and dry area to induce sweat.
The bathroom patrons used olive oil to cleanse themselves by applying it to their bodies and using a stencil to remove excess.
In a modern Spa, a caldarium is a room with a heated floor.