solarium

noun4

1. •
   (US, Canada) A room, rooftop, balcony, or terrace, used for its abundant sunlight, especially when used as a medical treatment.

   • My brother kept a health-lift a few years ago... and then he had a blue-glass solarium. — 1891
2. •
   (UK) Synonym of tanning salon, a room or business used for its sunlamps or tanning beds.
3. •
   An earthen structure constructed by certain ants for the purpose of brood incubation.
4. •
   (no longer used) Synonym of sundial.

   • […] this solarium being made for a different meridian […] — 1842