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.