The outer part (skin with a thickness of zero) of a body; can apply to structures, to micrometer-sized particles, or to extended-surface zeolites.

A subset of three-space consisting of those points whose cartesian coordinates x, y, and z satisfy equations of the form x = f (u,v ), y = g(u,v ), z= == h(u,v ), where f, g, and h are differentiable real-valued functions of two parameters u and v which take real values and vary freely in some domain.