A protrusion is something that sticks out or juts beyond a surface or boundary. When you press your tongue against the inside of your cheek, you create a visible protrusion on the side of your face. A rock formation that extends beyond a cliff face is a protrusion. The bump of a tree root pushing up through a sidewalk is a protrusion.

The word comes from the idea of protruding, which means extending outward. Protrusions can be natural, like the bony protrusions of your knuckles and elbows, or created, like a handle protruding from a drawer. They can be permanent features or temporary bumps.

Notice that protrusion suggests something that breaks an otherwise smooth or regular surface. If you're checking a wall before painting, you might run your hand along it, feeling for protrusions that need sanding down. In medicine, doctors examine patients for unusual protrusions that might signal broken bones or swelling.

The word often appears in technical or descriptive writing. An engineer might worry about a metal protrusion that could catch on something, while a geologist might study rock protrusions to understand how mountains formed. Whenever something pokes out, extends beyond, or creates a bump where things should be flat or even, you're looking at a protrusion.