Describing or involving the form of electromagnetic signal that is carried by a waveguide with a cross section that is small, or of the same order of size, compared with the wavelength of the electromagnetic wave. The wave will only propagate purely along the axis of the waveguide, with very little degradation apart from a possible loss of amplitude. This is especially valuable for systems using digital signals, which in general rely on the preservation of sharp edges in the signals. See also fibre optics, multimode.