Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy that determines the temperature of matter. The geothermal energy of the Earth's crust originates from the original formation of the planet and from radioactive decay of materials (in currently uncertain but possibly roughly equal proportions).The adjective geothermal originates from the Greek roots γη (geo), meaning Earth, and θερμος (thermos), meaning hot.