When a system of forces act on a body, it undergoes some deformation. This deformation per unit length is known as unit strain or simply a strain.
Mathematically
Strain = (Change in Length)/(Original Length)

Primary Strain or Linear Strain The deformation of the bar, per unit length in the direction of the force is known as primary or linear strain.
Secondary Strain or Lateral Strain Every direct stress is always accompanied by a strain in its own direction, and an opposite kind of strain in every direction, at right angles to it. Such strain is known as secondary or lateral strain.