In heterodyne reception, a frequency, other than that to which the receiver is tuned, which is such that the difference between this frequency and that of a local oscillator in the receiver is a frequency lying within the pass band of an intermediate-frequency amplifier in the receiver, in which it produces an unwanted response.
Reference: B.S. (modified)
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Centre of buoyancy
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM