To protrude means to stick out or extend beyond a surface or boundary. When something protrudes, it juts forward in a way that's noticeable, like a tree root protruding from the ground that might trip you on a hiking trail, or a nail protruding from a board that could scratch you if you're not careful.

The word often describes things that stick out farther than they should or in unexpected ways. A rock protruding from a cliff face gives mountain climbers something to grip. A dog's tongue might protrude from its mouth when it's hot and panting. After you eat a huge meal, your stomach might protrude more than usual.

You'll see this word in books describing physical features: “His ears protruded slightly from his head” or “The castle's towers protruded above the treeline.” Scientists use it too. A biologist might note how a turtle's head protrudes from its shell, or how certain fish have eyes that protrude from the sides of their heads.

The noun form is protrusion, meaning something that sticks out: “There's a strange protrusion on that tree trunk, maybe an old broken branch.”