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 trade it for goods needed, for example construction materials.
On the moon of Andoria, Andorian cities were built underground to take advantage of the moon's geothermal energy. (ENT: "The Aenar")
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")