# Dictionary:Elastic constants, elastic moduli

{{#category_index:E|elastic constants, elastic moduli}} Elasticity deals with deformations that vanish entirely upon removal of the stresses that cause them. For small deformations, Hooke's law holds and strain is proportional to stress. The passage of a low-amplitude seismic wave is an example.

The general 3x3x3x3 elasticity tensor relating stress and strain can be expressed as a 6x6 matrix, using Voigt notation (Figure H-7). In anisotropic media this matrix possesses up to 21 independent constants.
In **isotropic media**, where properties are the same measured in any direction, these reduce to 2 independent constants.
In polar anisotropic (**transversely isotropic media**), where properties are the same measured in two orthogonal directions but different in the third, these reduce to 5 independent constants (see also *Thomsen parameters*).

The "constants" are not really constant, since they vary with pressure and temperature, and with large stress, so they are more properly called **elastic moduli** (Figure E-6), or stiffnesses, to contrast them with compliances.