www.alphadictionary.com

An indoor pool.

The word has a New England source history. Traces back to Latin and to Greek. Apparently, Irish root of usage.

I was reading a history of The Cloisters in NYC, and they mention that the Billings estate, Tyron Hall featured a "natatorium".

I first thought it might mean an arena for babies. Glad I was wrong. Glad it is more in the direction of a vernal pool.

E

In Monroe, LA stands the now closed Swyaze Natatorium, built in the '30's. It's an outdoor pool and was free when I was a kid. Learned to swim and lifesave there. Thirty miles away inRuston is the LA Tech Natatorium, also closed while they decide what to do with it after new construction of two pools, indoor and outdoor. The first one was mostly indoor, but its pavilion had so much glass, you thought you were outdoor. Chief advantage of outdoor pools in LA is the water is WARM!