In engineering mechanics, deformation is a change in shape due to an applied force. This can be a result of tensile (pulling) forces, compressive (pushing) forces, shear, bending or torsion (twisting). Deformation is often described in terms of strain.In the figure it can be seen that the compressive loading (indicated by the arrow) has caused deformation in the cylinder so that the original shape (dashed lines) has changed (deformed) into one with bulging sides. The sides bulge because the material, although strong enough to not crack or otherwise fail, is not strong enough to support the load without change, thus the material is forced out laterally. Deformation may be temporary, as a spring returns to its original length when tension is removed, or permanent as when an object is irreversibly bent or broken., showing the relationship between stress (force applied) and strain (deformation) of a ductile metal.]]The concept of a rigid body can be applied if the deformation is negligible
Source:
http://en.wikipedia.org/wiki/Deformed