Within the elastic limit of a solid material, the deformation (strain) produced by a force (stress) of any kind is proportional to the force. If the elastic limit is not exceeded, the material returns to it original shape and size after the force is removed, other it remains deformed or stretched. The force at which the material exceeds its elastic limit is called 'limit of proportionality.' Discovered in 1676 by the UK scientist and inventor Robert Hooke (1635-1703). See also elastic modulus.