Deformation is when something changes shape, usually by being bent, stretched, squeezed, or twisted out of its original form. When you press down on a tennis ball with your thumb, the temporary dent you create is a deformation. When a metal bridge beam bends under the weight of heavy traffic, that bending is deformation.

Scientists and engineers study deformation carefully because understanding how materials change shape helps them design safer buildings, stronger machines, and better products. Some deformations are elastic, meaning the object springs back to its original shape when the force stops, like a rubber band after you stretch it. Other deformations are plastic or permanent, like when you bend a paperclip so many times that it stays crooked even after you let go.

The word can also describe changes in the earth itself. Earthquakes happen when underground rock layers undergo sudden deformation. Geologists study how mountains form through slow deformation over millions of years as massive plates of Earth's crust push against each other.

In medicine, doctors might notice bone deformation from an injury or disease. In everyday life, you might see deformation when your backpack straps leave temporary dents on your shoulders, or when a basketball slowly loses its perfectly round shape after years of use.