If a point moves with relative to another nearby point in an object, a strain occurs. Stated another way, a strain is caused when a gradient in the displacement field occurs in an object. An object that moves as a rigid body has no strain.

A displacement that can be described by a simple coordinate transformation does not produce stress and strain. Stated in other words: Displacement and strain is not the same thing. However, there must be a relation. Displacement changes the position of atoms - and so does strain.

Displacement refers to the change in position of a particle. When the displacement is very small, it is referred to as infinitesimal; when large, it is referred to as finite. Strain is also related to the displacement of particles from their original position to a new position.