In telecommunication, return loss is 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 Z ₁ is the impedance toward the source and Z ₂ is the impedance toward the load, and the vertical bars indicate magnitude.

Note 4: For dielectric media, e.g, optical fibers, see reflection loss (def #2).

Source: from Federal Standard 1037C and from MIL-STD-188