The ability of a radio receiver to discriminate against radiowaves that have a carrier frequency different from that to which the receiver is tuned (see tuned circuit). The selectivity is usually expressed on a graph that shows the power ratio E/E_o expressed in decibels against frequency, at specified values of the modulation factor or index and modulation frequency; E_o is the output power at the resonant frequency, f_o, and E the output power at frequency f for the same value of input power as that producing E_o.

The graph indicates the factor by which the input power must be increased, as the carrier frequency is varied from the resonant frequency to which the receiver is tuned, in order to maintain a constant output. It is usual to render any automatic gain control inoperative when selectivity curves are being obtained.