The reduction in power between any two points 1 and 2 in a telecommunication system given by the ratio P2/P1, where P1 and P2 are the powers at points 1 and 2 respectively and 2 is further away from the source of power than 1. For a given transmission path at a designated frequency the transmission loss is given by the ratio of the available power at the input of a receiver to the power transmitted from the output of the transmitter.
In any system where gain is introduced to the signal, the transmission gain is given by the ratio of the powers P2/P1. This represents the increase in power between the points 1 and 2 where 2 is again further from the source of power than 1.