Thinness is the state or quality of being thin. The word describes objects, materials, or living things that have little thickness, width, or body mass.

When talking about objects and materials, thinness often relates to how much space something takes up from one side to the other. The thinness of a sheet of paper makes it easy to store thousands of pages in a single book. The thinness of smartphone screens has made modern devices remarkably portable. Ice skaters must worry about the thinness of ice in early winter, since thin ice can crack under their weight.

For liquids and mixtures, thinness describes how watery or runny something is. If you add too much milk to your hot chocolate, you might complain about its thinness. A painter might adjust the thinness of their paint by adding water or solvent.

When describing people's bodies, the word means having relatively little body fat or muscle mass, though people often use more specific words like slender, lean, or skinny instead. Each of these words carries different feelings: slender sounds elegant, lean suggests fitness, while skinny can suggest someone is unhealthily thin.

The word can also describe abstract qualities. The thinness of an excuse means it's weak and unconvincing. When a crowd spreads out, you notice the thinness of people in certain areas.