1. . 
The permanent deformation evident in rocks and other solid bodies that have experienced a sufficiently high applied stress. A change in shape, such as folding, faulting, fracturing, or change, generally a reduction, in volume are common examples of strain seen in rocks. Strain can be described in terms of normal and shear components, and is the ratio of the change in length or volume to the initial length or volume.
For more on strain: Means WD: Stress and Strain. New York, New York, Springer-Verlag, 1976.