In a deformed body, restoring force is set up within the body which tends to bring the body back to the normal position. The magnitude of these restoring force depends upon the deformation caused. This restoring force per unit area of a deformed body is known as stress.
Stress = (restoring force/area) N m2
Its dimensional formula is ML-1T-2.
Due to the application of deforming force, length, volume or shape of a body changes. Or in other words, the body is said to be strained. Thus, strain produced in a body is defined as the ratio of change in dimension of a body to the original dimension.
Strain = change in dimension / original dimension
Strain is the ratio of two similar quantities. Therefore it has no unit.