The term spa is associated with water treatment which is also known as balneotherapy. Spa towns or spa resorts (including hot springs resorts) typically offer thermal or mineral water for drinking and bathing. They also offer various health treatments. The belief in the curative powers of mineral waters goes back to prehistoric times. Such practices have been popular world-wide, but are especially wide-spread in Europe and Japan. Day spas are also quite popular, and offer various personal care 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, perhaps related to the Latin word "spargere" meaning to scatter, sprinkle or moisten.

Since medieval times illnesses caused by iron deficiency were treated by drinking chalybeate (iron-bearing) spring water (in 1326, the ironmaster Collin le Loup claimed a cure, when the spring was called Espa, a Walloon word for "fountain").

In 16th century England the old Roman ideas of medicinal bathing were revived at towns like Bath, and in 1571 William Slingsby who had been to the Belgian town (which he called Spaw) discovered a chalybeate spring in Yorkshire. He built an enclosed well at what became known as Harrogate, the first resort in England for drinking medicinal waters, then in 1596 Dr Timothy Bright called the resort The English Spaw, beginning the use of the word Spa as a generic description rather than as the place name of the Belgian town. At first this term referred specifically to resorts for water drinking rather than bathing, but this distinction was gradually lost and many spas offer external remedies.

It is commonly claimed, in a commercial context, that the word is an acronym of various Latin phrases such as "Salus Per Aquam” or "Sanitas Per Aquam" meaning "health through water". This is very unlikely: the derivation doesn't appear before the early 21st century and is probably a "backronym" as there is no evidence of acronyms passing into the language before the 20th century; nor does it match the known Roman name for the location.