The ratio of the transmitted field strength to the incident field strength of an electromagnetic wave when it is incident upon an interface surface between media with two different refractive indices. 2. In a transmission line, the ratio of the amplitude of the complex transmitted wave to that of the incident wave at a discontinuity in the line. 3. The probability that a portion of a communications system, such as a line, circuit, channel or trunk, will meet specified performance criteria. Note: The value of the transmission coefficient is inversely related to the quality of the line, circuit, channel or trunk.