Whenever a material is loaded within elastic limit, stress is directly proportional to strain

The ratio of linear stress to the linear strain is called as modulus of elasticity (Young's Modulus)

The unit of modulus of elasticity is same as those of stress, pressure and modulus of rigidity.

The deformation of the bar, per unit length in the direction of the force (i.e. δl/l) is known as primary strain or linear strain.

When a change in length takes place, the strain is known as linear strain.