The ratio, at the junction of a transmission line and a terminating impedance or other discontinuity, of the amplitude of the reflected wave to the amplitude of the incident wave. Note 1: Return loss is usually expressed in dB. Note 2: Return loss is a measure of the dissimilarity between impedances in metallic transmission lines and loads, or between refractive indices in dielectric media, e.g., optical fibers. Note 3: In a metallic transmission line, return loss is given by where Z1 is the impedance toward the source and Z2 is the impedance toward the load, and the vertical bars indicate magnitude. Note 4: For dielectric media, e.g., optical fibers, see reflection loss.