Geo is a prefix meaning “earth” or “related to the Earth.” It comes from the ancient Greek word for earth and appears at the beginning of many words you'll encounter in science and everyday life.

When you see geo at the start of a word, you know it has something to do with our planet. Geography is the study of Earth's lands, features, and peoples. Geology examines Earth's rocks, minerals, and how the planet's structure changes over time. Geometry might seem like an exception, but it originally meant “earth measurement” because ancient people used it to measure land and fields.

You'll find geo in modern words too. A geothermal power plant uses heat from inside the Earth to generate electricity. When you use a geocache app to find hidden treasures, you're using geographic coordinates to locate them. Some apps even add geo to their names to show they work with maps and locations, like geotagging photos to remember where you took them.

Once you recognize this prefix, you'll spot connections everywhere: geophysics, geopolitics, geodesic. Each time, that geo at the beginning points you toward something involving planet Earth.