Deformation is a physical change where an object (like a spring) is compressed or stretched.

When stretched a small amount by a deforming force, groups of atoms are moved slightly away from each other, but can easily snap back into position. This is an elastic deformation where changes are fully reversible.

If stretched far enough, the small groups of atoms will rearrange into a different shape or separate into smaller groups. This means they will not be able to snap back to their ordered position when the deforming force is removed. This creates a permanent deformation known as plastic deformation.

A cartoon of groups of atoms stretching.

Figure 1 - A spring stretching elastically, then plastically, leading to permanent deformation.