The term spa is associated with water treatment, spa towns or spa resorts (including hot springs resorts) typically offer various health treatments.
The term is derived from the name of the town of Spa, Belgium, whose name is known back to Roman times, when the location was called Aquae Spadanae, sometimes incorrectly connected to the Latin word "spargere" meaning to scatter, sprinkle or moisten. It had springs of waters with reputed healing properties.
By 1626 we have a citation for an English Spa and by 1777 the name of the town of Spa became truly generic so that any town with a hot spring where you could soak your ills away was called a spa.
It wasn't until about 1960 that people began opening health clubs and calling them spas without having a hot-springs on the premises.
After that everyone wanted one; in 1974 you could get an aerated bathtub in your own house and call it a spa.