A warm spring or hot spring is a place where warm or hot groundwater issues from the ground on a regular basis for at least a predictable part of the year, and is significantly above the ambient ground temperature (which is usually around 55~57°F or 13~14°C in the eastern United States). The water is heated by geothermal heat, or heat generated from the interior of the Earth. This occurs in various "hot spots", where magma or other mantle material is close to the surface. If the water becomes so heated that it builds steam pressure and erupts in a jet above the surface of the Earth, it is called a geyser; if the water only reaches the surface in the form of steam, it is called a fumarole; and if the water is mixed with mud and clay, it is called a mud pot.
Warm springs are sometimes the result of hot and cold springs mixing but may also occur outside of geothermal areas, such as Warm Springs, Georgia (frequented for its therapeutic effects by polio-stricken U.S. President Franklin D. Roosevelt, who built the Little White House there). Because heated water can hold more dissolved solids and gases, warm and especially hot springs also often have a very high mineral content, containing everything from simple calcium to lithium, and even radium. Because of both the folklore and the proven medical value some of these springs have, they are often popular tourist destinations, and locations for rehabilitation clinics for those with disabilities.  
At least three United States national parks feature hot springs:
- Yellowstone National Park in Wyoming (and Idaho/Montana) 
- Hot Springs National Park in Arkansas 
- Lassen Volcanic National Park in California