Deformation is the change in shape or size of an object due to applied forces. It can be elastic (reversible) or plastic (permanent).
The force per unit area within materials that arises from externally applied forces.
The amount of deformation experienced by the body in the direction of the applied force divided by the initial dimensions of the body.
$$\text{Young's Modulus}$$: $$\text{A measure of the stiffness of a solid material, defined as stress over strain.}$$