Hydro- is a prefix meaning “water.” When you see hydro- at the start of a word, it signals that water plays a key role in whatever that word describes.

A hydroelectric dam generates electricity from flowing water. Hydration means getting enough water into your body. A hydroponic garden grows plants in water instead of soil. Hydrogen is a chemical element whose name literally means “water-former” because it combines with oxygen to form water.

You'll find hydro- everywhere water matters: in science class learning about the hydrosphere (all the water on Earth), in geography studying hydropower (energy from water), or even at the pool talking about hydrodynamics (how objects move through water). When you're thirsty and need to rehydrate, you're literally putting water back into your body. The prefix works like a helpful sign saying “water is involved here.”