* Geo (γη) - meaning "earth"

* Therme (θερμή) - meaning "heat"

So, geothermal literally means "earth heat." It refers to the heat within the Earth's interior. This heat is generated from:

* Radioactive decay: The decay of radioactive elements like uranium, thorium, and potassium within the Earth's mantle and crust produces heat.

* Residual heat: From the Earth's formation billions of years ago, some heat remains trapped within the planet.

This internal heat manifests in various ways, including:

* Volcanic eruptions: Molten rock (magma) rises to the surface, releasing heat.

* Geysers and hot springs: Water heated by underground geothermal activity erupts or flows to the surface.

* Geothermal gradients: The temperature of the Earth's crust increases with depth.

Therefore, "geothermal" refers to the heat energy present within the Earth, and the various ways it is released or utilized.