When a system of forces or loads act on a body, it undergoes some deformation. This deformation per unit length is known as unit strain or simply a strain. It is denoted by a Greek letter epsilon (ε). Mathematically,
Strain, ε = δl / l or δl = ε.l
δl = Change in length of the body,
and l = Original length of the body.
A textbook of Machine Design by R.S.Khurmi and J.K.Gupta