Plain English Meaning
A small radio beacon placed at one of the marker beacons on an ILS approach. The pilot tunes it on the ADF and uses it to find the approach course, hold, or back up the ILS.
Full Definition
A low-power, low- or medium-frequency (L/MF) radio beacon installed at the site of the outer or middle marker of an instrument landing system (ILS). It transmits an identification signal that can be received by an aircraft's automatic direction finder (ADF), allowing the pilot to use it for navigation, , and as an aid during the ILS approach. The is typically usable within at least 15 , and the within at least 5 nautical miles.