1. (in transmission) The ratio of the power that must be supplied to a reference antenna compared to the power supplied to the antenna under consideration in order that they produce exactly similar field strengths at the same distance and in the same specified direction (usually the direction of maximum radiation).

2. (in reception) The ratio of the signal power produced at the receiver input by the given antenna to that produced by a reference antenna under similar receiving conditions.

In both cases the reference antenna must be specified.