Plaster is a soft, paste-like building material that hardens when it dries, used to create smooth walls and ceilings inside buildings. Workers spread wet plaster over rough surfaces like wooden strips or concrete blocks, smoothing it with flat tools. As it dries, it becomes hard and creates the smooth white walls you see in many rooms.

For centuries, builders made plaster by mixing lime, sand, and water. Today, most plaster contains gypsum, a soft mineral that's ground into powder and mixed with water. The Egyptians used plaster to create smooth surfaces inside the pyramids over 4,000 years ago.

Plaster also refers to the stiff bandages doctors wrap around broken bones. These plaster casts start soft but harden into a protective shell that keeps the bone still while it heals. If you've ever signed a friend's cast after they broke their arm, you've written on dried plaster.

You might also plaster something with stickers or posters, meaning to cover it completely. If your walls are plastered with drawings, they're covered from top to bottom. The word suggests thorough coverage, like how actual plaster covers every bit of a wall.