also: スパー
spa (mineral spring)
Refers specifically to a natural hot spring or mineral spring, often used in travel or resort contexts.
There are many natural spas in this area.
spa (beauty/relaxation facility); health spa
Refers to a commercial establishment offering beauty treatments, massages, weight loss programs, etc., not necessarily a natural spring.
I got a massage at a spa on the weekend.
We went to a spa at a hot spring resort over the weekend.
I got a massage at the hotel spa.
At the end of the trip, I relaxed at the spa.