caldarium

noun3

1. •
   In modern spas, a room with a hot floor.
2. •
   (historical) In Roman baths, the hottest room, with a plunge-pool. It preceded the tepidarium and frigidarium.
3. •
   An intermediate or warm greenhouse.

   • There is a pretty large glass-house, one-half being a caldarium, the other a tepidarium — 1851