Geothermal energy was a scientific term used to describe the amount of energy a planet could produce from internal heat generated by the core. Some planets with surplus energy traded it for needed goods, for example construction materials.
The planet Deneb IV had abundant reserves of geothermal energy. The energy was used to hold a space creature captive, a being able of forming itself into any shape the captors wanted – in this case, Farpoint Station. Eventually, the crew of the USS Enterprise-D freed this creature from its tormentors. (TNG: "Encounter at Farpoint")
On an unnamed asteroid in the Delta Quadrant, a group of Talaxians established a colony where they dug several tunnels and used the asteroid's geothermal energy to melt the ice that covered most of the outer surface, which provided them with oxygen and water. That technology was later given to Nocona and was converted into fuel used to power his ships. (VOY: "Homestead")