In superheterodyne reception, the frequency which is derived by mixing the signal-carrying frequency with the local oscillator frequency. If there be more than one such mixing process the successive intermediate frequencies are known as the first, second, etc. intermediate frequency.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
nertia of the water plane
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM