In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness", has implications in physics and seismology. Typical "smooth surfaces", even those polished to a mirror finish, are still not truly smooth on a microscopic scale. They are rough, with sharp, rough, or rugged projections, termed "asperities". Surface asperities exist across multiple scales, often in a self-affine or fractal geometry. The fractal dimension of these structures has been correlated with the contact mechanics exhibited at an interface in terms of friction and contact stiffness.