1. The value of some quantity associated with the resultant field produced by incident and reflected waves at a given point in a transmission medium divided by the corresponding quantity in the incident wave.

2. The ratio of transmitted to incident energy flux or flux of some other quantity at a discontinuity in a transmission medium; for sound waves, it is called the sound transmission coefficient.

3. The ratio of the transmitted flux of some quantity to the incident flux for a substance of unit thickness.

See <<penetration probability>>