Shear Modulus (also known as Modulus of Rigidity, $G$ or $\eta$) characterizes a material's resistance to a shape change while keeping the volume constant. It is the ratio of tangential (shear) stress to shear strain: $G=\frac{\text{Shear Stress}}{\text{Shear Strain}}=\frac{F_{tan}/A}{\theta }$ where $\theta$ is the angle of shear (displacement divided by height). Liquids and gases cannot support shear stress, so their shear modulus is effectively zero.