A hot spring is a spring that is produced by the emergence of geothermally heated groundwater from the earth's crust. There are hot springs all over the earth, on every continent and even under the oceans and seas.

There is no universally accepted definition of a hot spring. For example, one can find the phrase hot spring defined as:

• any geothermal spring
• a spring with water temperatures above its surroundings
• a natural spring with water temperature above body temperature – normally between 36.5 °C (97.7 °F) and 37.5 °C (99.5 °F)
• a natural spring with warm water above body temperature
• a thermal spring with water warmer than 36.7 °C (98.1 °F)
• a natural spring of water greater than 21.1 °C (70.0 °F) (synonymous with thermal spring)
• a natural discharge of groundwater with elevated temperatures
• a type of thermal spring in which hot water is brought to the surface. The water temperature of a hot spring is usually 6.5 °C (11.7 °F) or more above mean air temperature. Note that by this definition, "thermal spring" is not synonymous with the term "hot spring".
• a spring whose hot water is brought to the surface (synonymous with a thermal spring). The water temperature of the spring is usually 8.3 °C (14.9 °F) or more above the mean air temperature.
• a spring with water above the core human body temperature – 36.7 °C (98.1 °F).
• a spring with water above average ambient ground temperature, a definition favored by some.
• a spring with water temperatures above 50 °C (122 °F)

The related term "warm spring" is defined as a spring with water temperature less than a hot spring by many sources, although Pentecost et al (2003) suggest that the phrase "warm spring" is not useful and should be avoided.The US NOAA Geophysical Data Center defines a "warm spring" as a spring with water between 20 °C (68 °F) and 50 °C (122 °F). In Japan, the ♨ symbol is commonly used on maps to denote a hot spring.