Plain English Meaning
A small radio beacon placed at the middle marker position on an ILS approach. It does two jobs at once: it acts as the middle marker, and it gives a signal pilots can home in on with their ADF.
Full Definition
A low-power non-directional beacon (NDB) installed at the site of an ILS middle marker, transmitting a homing signal that pilots can track with their ADF receiver. It serves both as the middle marker for the ILS approach and as a standalone navigation aid for orienting and approaching the runway.
Context Anchor
Seen in instrument approach equipment discussions and on some approach charts for older .