'Geyser basin' is a general, and only semi-formal, term for an area of geothermal activity including multiple geysers and related features.
Geysers are the result of surface water seeping down through cracks to rock that has been heated by magma that is fairly near the surface. Because of this, they are generally found in valleys, calderas, and other comparatively low-lying areas -- although this may be low-lying only in comparison to the higher mountains around the basin. Even given these conditions, geysers are a fairly rare phenomenon, as they require specific 'plumbing' in order to spout. So it is generally the case that if you have multiple geysers, you will also have other, more common, geothermal features.
While every geyser basin is different, they often include hot springs and heated pools, limestone terraces and sinter mounds, mud pots, and/or fumaroles. Areas labeled as basins generally range from a bit less than a kilometer in diameter to two or three kilometers. However, these basins often appear in clusters and with uncertain boundaries, including in strips of land bordered by lakes or rivers.
Some of the better known geyser basins and geothermal areas include:
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Yellowstone National Park, USA, contains a number of geyser basins (at least ten open to the public), and has the greatest number of geysers and related features.
- Haukadalur, Iceland has some of the most thoroughly managed geysers, some of which are carefully fed water and treated with soap to stimulate eruptions; some have been drilled to clear blocked fissures, and others may be cleared in the future. This sort of thing is not too uncommon in tourist parks, but is generally not done with such disregard for the natural functioning of the geysers.
- Valley of Geysers (Долина гейзеров) on Russia's Kamchatka Peninsula, is Russia's only geyser basin, and one of the hardest to reach. It is accessible only by helicopter, and visitors are limited. In 2007 a massive mud flow buried a large section of the basin, but it has been recovering nicely.
- Taupo Volcanic Zone, North Island, New Zealand, is quite extensive and includes many volcanic vents. The Waimangu Volcanic Rift Valley includes the Inferno Crater Lake, which contains the largest 'geyser-like feature' in the world, namely, an underwater geyser.
- El Tatio, Andes, Chile, is the highest elevation geyser basin (4,320 meters above sea level).
Some sources will refer to the fissure from which a geyser erupts as a geyser basin; in these cases, the larger area of geothermal activity may be referred to as a geyser field.