A sanatorium is a kind of hospital where people stay a long time to get better after a serious illness, with rest, clean air, and a lot of care — not a clinic you visit for one afternoon. For many years the picture in English was a quiet building in the mountains or by the sea, especially for people with a lung disease called TB. Doctors believed rest would do the healing. Those big old sanatoriums have mostly closed. Today the word feels a little old. Families more often say a recovery hospital, rehab, or just hospital. American English often writes sanitarium, which is the same broad idea; though actually some American speakers used sanitarium for a health farm or, in older times, a place for mental illness, so the two spellings are close cousins, not perfect twins. You will meet sanatorium in history, novels, and old signs more than on a modern hospital door.