Plain English Meaning
A small radio beacon located at the middle marker of an ILS approach. The pilot can home in on it using the ADF needle to help line up with the runway during an instrument approach.
Full Definition
A low-power, low/medium frequency (L/MF) non-directional radio beacon installed at the site of the middle marker of an Instrument Landing System (ILS), used as an aid for navigating to the final approach course. It transmits a continuous identification signal and can be received by a standard ADF (Automatic Direction Finder) in the .