Elasticity
Elasticity is that property of a material that allows it to return to its original shape after being deformed. Referencing the diagram to the right, if you apply a load F to a rod of cross-sectional area A, an elongation of Δ1 takes place in the original length (1).
The applied force creates stress in the rod, and the rod responds to the stress by deforming. Since Δ1 increases in response to increasing applied force, you can see that the strain on a aterial is a result of, and directly proportional to, the stress applied.