The Shear Modulus, also known as the modulus of rigidity, is a measure of a material's ability to resist deformation when a shear force is applied. It quantifies how much a material will deform when forces are applied parallel to its surface. For example, if you push the top of a rubber block sideways, the shear modulus helps determine how much the block will change shape. This property is crucial in engineering and materials science, as it influences how materials behave under stress. High shear modulus values indicate that a material, like steel, is stiff and resists deformation, while lower values suggest that materials, such as foam, are more flexible and can easily change shape.