In telecommunications, return loss is the loss of power in the signal returned/reflected by a discontinuity in a transmission line. This discontinuity can be a mismatch with the terminating load or with a device inserted in the line. It is usually expressed as a ratio in decibels (dB);
RL(dB) = 10Log Pi/Pr
Where RL(dB) is the return loss in dB, Pi is the incident power and Pr is the reflected power.
Return loss is related to standing wave ratio (SWR). Increasing return loss corresponds to lower SWR. Return loss is a measure of how well devices or lines are matched. A match is good if the return loss is high. A high return loss is desirable and results in a lower insertion loss.
Being a logarithmic function return loss is used in modern practice by engineers in preference to SWR because it has better resolution for small values of reflected wave.
A return loss plot 40 meter Inverted V dipole
And a plot of my 20 meter beam
These plots are typical of correctly operating Amateur antennas the plots show that return loss decreases at band edges indicating that the antennas have reasonable Q and low losses.
If the plots had a return loss that was constant over the entire band we could be certain that losses were high and that they would make good dummy loads!