A gill is an organ that allows fish and many other water-dwelling creatures to breathe underwater. While you breathe air through your lungs, a fish breathes by pulling water through its gills, which extract oxygen dissolved in the water. If you watch a fish carefully, you'll see flaps on the sides of its head opening and closing: those are its gill covers, working like tiny pumps to keep water flowing over the delicate gill tissues inside.

Gills work remarkably well. They let fish, sharks, octopuses, and countless other ocean animals get all the oxygen they need without ever surfacing for air. Even tadpoles have gills before they develop lungs and become frogs. The invention of scuba diving equipment, which lets humans breathe underwater, was inspired partly by studying how efficiently gills work.

The word gill (pronounced like “jill”) also refers to a small unit of liquid measurement equal to one-quarter of a pint, though you'll rarely encounter this meaning outside of old recipes or historical contexts.