I think you might be underestimating what it means to be fluent in a language. The most important criterion in my opinion is whether you are able to freely speak in that language without doing any on the fly translating.
Geothermal energy is thermal energy within the Earth due to the movement of the Earth's particles. Most of this geothermal energy is contained within the core of the Earth. This geothermal energy is renewable energy as it is energy derived from a natural resource and it can be replenished.
Thermal energy results in an object or a system having a temperature that can be measured. Thermal energy can be transferred from one object or system to another in the form of heat. Geothermal energy is thermal energy within the Earth due to the movement of the Earth's particles. Most of this geothermal energy is contained within the core of the Earth.