The deformation produced in a material by stress is called strain.  When stress is applied on a material, there is a change in dimensions.  For instance, if a steel rod is stretched, its length increases.  This change in dimensions is called strain.

True Strain
True Strain is the natural logarithm of the ratio of the final length to the original length.

True Strain = Ln (Final Length/Original Length)

Engineering Strain
Engineering Strain expresses strain as the ratio of the change in length to the original length.

Engineering Strain = Change in Length/ Original Length