The term spa is associated with water treatment which is also known as balneotherapy. According to the International Spa Association, spa is an entity devoted to enhancing overall well-being through a variety of professional services that encourage the renewal of mind, body and spirit.

The term is derived from the name of the town of Spa, Belgium, where since medieval times illnesses caused by iron deficiency were treated by drinking chalybeate (iron bearing) spring water. 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.

There are various stories about the origin of the name. A Belgian spring of iron bearing water was called Espa from the Walloon language term for "fountain", and was used in 1326 as a cure by an iron master with such success that he founded a health resort which developed into the town. It is also suggested that the term Espa may be derived from the name of the resort, and that its source could be the Latin word "spagere" meaning to scatter, sprinkle or moisten. It has been suggested, with no evidence, that the word is an acronym of various Latin phrases such as "Salus Per Aquam” or "Sanitas Per Aquam" meaning "health through water". These are "backronyms" — back-formed acronyms. Acronyms arose in the twentieth century, and were not used in classical times.

There are several types of spas:

• Club spa: a facility whose primary purpose is fitness and which offers a variety of professionally administered spa services on a day-use basis.
• Cruise ship spa: a spa aboard a cruise ship providing professionally administered spa services, fitness and wellness components and spa cuisine menu choices.
• Day spa: a spa offering a variety of professionally administered spa services to clients on a day-use basis.
• Dental spa: a facility under the supervision of a licensed dentist that combines traditional dental treatment with the services of a spa.
• Destination spa: a destination spa is a facility with the primary purpose of guiding individual spa-goers to develop healthy habits. Historically a seven-day stay, this lifestyle transformation can be accomplished by providing a comprehensive program that includes spa services, physical fitness activities, wellness education, healthful cuisine and special interest programming.
• Medical spa: a facility that operates under the full-time, on-site supervision of a licensed health care professional whose primary purpose is to provide comprehensive medical and wellness care in an environment that integrates spa services, as well as traditional, complimentary and/or alternative therapies and treatments. The facility operates within the scope of practice of its staff, which can include both Aesthetic/Cosmetic and Prevention/Wellness procedures and services.
• Mineral springs spa: a spa offering an on-site source of natural mineral, thermal or seawater used in hydrotherapy treatments.
• Resort/hotel spa: a spa owned by and located within a resort or hotel providing professionally administered spa services, fitness and wellness components and spa cuisine menu choices. In addition to the leisure guest, this is a great place for business travelers who wish to take advantage of the spa experience while away from home.
  

Spa medication or equipment:
Spa (mineral water), from the sources in Spa.
TrimSpa weight loss pills
A soda fountain, in United States usage.
A hot tub, in United States usage.
A small bath in which feet can be washed and massaged is commonly sold as a foot spa.