The Shear Modulus or modulus of rigidity, denoted by $G$, or sometimes $S$ or $\mu$, is defined as the ratio of shear stress to the shear strain.
Applicable units are those of quantitykind:ShearModulus
http://en.wikipedia.org/wiki/Shear_modulus
$G = \frac{\tau}{\gamma}$, where $\tau$ is the shear stress and $\gamma$ is the shear strain.
http://www.iso.org/iso/catalogue_detail?csnumber=31889