In a waveguide, a time-varying field having an amplitude that decreases monotonically as a function of transverse radial distance from the waveguide, but without an accompanying phase shift. Note 1: The evanescent field is coupled, i.e., bound, to an electromagnetic wave or mode propagating inside the waveguide. Note 2: The evanescent field is a surface wave. Note 3: In fiber optics, the evanescent field may be used to provide coupling to another fiber.