What does "elasticity" mean?

Elasticity: Power of resistance to, or recovery from, depression or overwork. Coefficient of elasticity, the quotient of a stress (of a given kind), by the strain (of a given kind) which it produces; -- called also coefficient of resistance. -- Surface of elasticity (Geom.), the pedal surface of an ellipsoid (see Pedal); a surface used in explaining the phenomena of double refraction and their relation to the elastic force of the luminous ether in crystalline media.