A strain is the response of a system to an applied stress. When a material is loaded with a force, it produces a stress, which then causes a material to deform.

Strain is simply the measure of how much an object is stretched or deformed. Strain occurs when force is applied to an object. Strain deals mostly with the change in length of the object.

Strain, denoted by the letter (ε) “epsilon,” is defined as the relative deformation or change in shape that occurs in a material in response to an external force or load. It is a measure of the degree to which a material has changed shape due to stress.

Strain is the ratio for change of shape or size to the original shape or size. It is expressed in number as it doesn't have any dimensions. Since strain defines the relative change in shape and it is a dimensionless quantity. A body can experience two types of strain depending upon the stress application.

When a force is applied to the surfaces of a solid material, it develops a stress - a pattern of forces between the atoms in the solid. In response to a stress, the material develops a change of shape, or strain. When the stress is not too large, the strain is reversible (elastic).