Image rejection ratio

Image rejection ratio: In reception using heterodyning in the tuning process, the ratio of (a) the intermediate-frequency (IF) signal level produced by the desired input frequency to (b) that produced by the image frequency.

Note 1: The image rejection ratio is usually expressed in dB. Note 2: When the image rejection ratio is measured, the input signal levels of the desired and image frequencies must be equal for the measurement to be meaningful. Synonym: image frequency rejection ratio.

Source: from Federal Standard 1037C and from MIL-STD-188