Geysir (sometimes known as The Great Geysir) is the name for a hot spring which erupts in a very special way. Such springs are known as geysers in English. This particular geyser is in the Haukadalur valley, Iceland. It is the oldest known geyser and one of the world's most impressive examples of the phenomenon. The English word geyser to describe a spouting hot spring derives from Geysir (which itself is derived from the Icelandic verb gjósa meaning to erupt. The English verb gush is probably related to that word). Geysir lies on the slopes of Laugarfjall hill at , which is also the home to Strokkur geyser about 400 metres south.
Eruptions at Geysir can hurl boiling water up to 60 metres in the air. However, eruptions may be infrequent, and have in the past stopped altogether for years at a time.