It is often used in words related to the Earth, its features, or its study:
* Geography: The study of the Earth's surface, including its physical features, human populations, and natural phenomena.
* Geology: The study of the Earth's composition, structure, and history.
* Geophysics: The study of the Earth's physical properties and processes.
* Geodesy: The study of the Earth's shape, size, and gravity field.
* Geothermal: Relating to the heat within the Earth.
* Geopolitics: The study of the interaction between geography and politics.
So, whenever you see "geo" in a word, it's a good clue that the word is related to the Earth.