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
geothermal gradient, which is the difference in temperature between the core of the planet and its surface, drives a continuous
conduction of thermal energy in the form of
heat from the core to the surface. The adjective
geothermal originates from the Greek roots
γη (ge), meaning earth, and
θερμος (thermos), meaning hot.