TIA's Glossary of Telecommunication Terms
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return loss
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. (188) 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).