The prefix geo- means “earth” or “related to the Earth.” It comes from ancient Greek and appears at the beginning of many English words about our planet.

Geography is the study of Earth's lands, features, and populations. Geology examines Earth's rocks, minerals, and how the planet formed over billions of years. Geometry literally means “measuring the earth,” which is what ancient surveyors did when they measured land and distances, though today geometry means the study of shapes, angles, and spatial relationships.

You'll find geo- in many other words: a geothermal pool uses heat from inside the Earth. Geocaching is a treasure-hunting game using GPS coordinates on Earth's surface. A geode is a hollow rock with crystals inside.

When you see geo- at the start of a word, think “earth” and you'll have a clue about the word's meaning. This prefix connects hundreds of English words to our home planet.