A geyser is a type of hot spring that erupts periodically, ejecting a column of hot water and steam into the air. The name geyser comes from Geysir, the name of an erupting spring at Haukadalur, Iceland; that name, in turn, comes from the Icelandic verb gjósa, "to gush".

The formation of geysers requires a favourable hydrogeology which exists in only a few places on Earth, and so they are fairly rare phenomena. About 1,000 exist worldwide, with about half of these in Yellowstone National Park, U.S. (Glennon, J.A. 2005). Geyser eruptive activity may change or cease due to ongoing mineral deposition within the geyser plumbing, exchange of functions with nearby hot springs, earthquake influences, and human intervention (Bryan, T.S. 1995).

Erupting fountains of liquefied nitrogen have been observed on Neptune's moon Triton. These phenomena are also often referred to as geysers. On Triton, the geysers appear to be driven by solar heating instead of geothermal energy. The nitrogen, liquefied by a kind of greenhouse effect, may erupt to heights of 8 km.