Sanitarium is the usual American spelling of sanatorium: a place, often in the countryside or the mountains, where people used to stay for weeks or months to get better from a long illness, especially tuberculosis. The word feels dated now; we more often say hospital, clinic, or rehab. In American history it was also used for health resorts — the Kellogg family ran a famous one — and, more darkly, for some mental institutions. Though actually, the two spellings are not a perfect twin in every old text: some American writers treated sanitarium as the wellness-spa word and sanatorium as the TB-hospital word, but for learners they are the same headword with an American -i-. It is not sanitizer, and it is not a place that makes you sane, even though the letters sit close.