Return loss

It is expressed in dB by taking the ratio of the reflected wave to the input signal, and is also called return loss. The sign is negative and the larger the absolute value, the higher the performance (less reflection).
Stabilization of characteristic impedance is the key to improving performance, and in recent years, it has become necessary to consider the behavior of minute parts, especially in response to high-speed transmission.
Note that VSWR (Voltage Standing Wave Ratio) expresses return loss in a different dimension, but has the same essential meaning (it can be converted uniquely).