Roughness is the quality of having an uneven, bumpy, or coarse surface rather than being smooth. When you run your hand along a brick wall, that scratchy, irregular feeling is roughness. Sandpaper has roughness that helps it smooth wood. A cat's tongue has roughness that helps it groom its fur and lap up water.

Roughness can describe textures you touch, but also sounds, movements, or even how people treat each other. A singer with a rough voice has a gravelly, raspy sound rather than a smooth, clear tone. If you're learning to ice skate, your movements might have a certain roughness until you develop more skill and grace. When someone speaks to you with roughness, they're being harsh or unkind rather than gentle.

In science and engineering, roughness matters a lot. The roughness of airplane wings affects how air flows over them. The roughness of a basketball gives players a better grip. Sometimes roughness helps, like the tread on your shoe sole that keeps you from slipping. Other times, engineers work hard to reduce roughness, like polishing telescope mirrors to be as smooth as possible so they can see distant stars clearly.