In materials science, asperity, defined as "unevenness of surface, roughness, ruggedness" (OED, from the Latin asper — "rough"), has implications (for example) in physics and seismology. Smooth surfaces, even those polished to a mirror finish, are not truly smooth on an atomic 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 and friction exhibited at an interface.