Strain is the deformation of a material from stress. Strain is simply a ratio of the change in length to the original length. Deformations that are applied perpendicular to the cross section are normal strains, while deformations applied parallel to the cross section are shear strains.
There is no unit for strain.

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.

The deformation of the bar per unit length in the direction of the force is known as linear strain.

The ratio of change in length to original length is called Linear strain.

The ratio of lateral strain to linear strain is called Poisson's ratio.