Geothermal energy is the internal heat of a planet or other large planetary body, this energy can often be harvested and converted for other uses such as electricity production.
On Andoria, cities are built underground to take advantage of the moon's geothermal energy. (ENT episode: "The Aenar")
The Tigans used geothermic energy to power a containment field which held a microscopic black hole which in turn produced power for the entire planet. (TNG comic: "History Lesson")
The planet Deneb IV had abundant supplies of geothermal energy which in the 2360s was used to hold a Star-jelly captive, forcing it to take on the form of Farpoint Station. (TNG episode: "Encounter at Farpoint")