Thinner is a liquid used to make paint less thick so it flows and spreads more easily. When paint sits in a can, it can become gloppy and hard to work with. Adding thinner makes it smoother and easier to brush or spray onto surfaces. Different types of paint need different thinners: oil-based paints use mineral spirits or turpentine as thinners, while water-based paints use plain water.

Artists use thinner to clean their brushes and to create different effects in their paintings. A painter might add thinner to create a wash, which is a very light, transparent layer of color. Professional painters adjust how much thinner they add depending on whether they're using a brush, roller, or spray gun.

The word also works as a comparative adjective meaning more thin than something else. A thinner piece of paper tears more easily than a thick one. If someone gets thinner, they weigh less than before. Ice on a pond becomes thinner as it melts in spring, so people should never walk on ice unless a responsible adult confirms it's safe.