When we apply a stress to a material we get a strain. But how much? The amount of strain will be proportional to the modulus of elasticity also known as Young's modulus or the materials stiffness. Yount's modulus is the slope of the stress vs strain curve during the elastic deformation region. Most materials have linear elastic regimes, some have non-linear elastic regimes. Ceramics have higher stiffness than metal and these are higher than polymers. Stiffness is proportional to the slope of the interatomic force vs interatomic separation distance evaluated at the equilibrium bond distance.