Return loss is signal attenuation caused by impedance variations in the structure of a cable or associated connection parts. These variations cause the signal to reflect (return) back to the source. At lower frequencies, return loss is a minor effect; at frequencies above 50 MHz, it can have a significant effect; and at frequencies used for high-definition video, 1500 MHz and higher, it can be a major, even critical, factor. For this reason it is suggested that return loss be a key consideration in the measurement and verification of HD system performance.