Plain English Meaning
A small radio beacon placed at the start of the final approach path on an ILS. The aircraft's direction-finding equipment can point straight at it, helping the pilot line up with the runway and confirm they have reached the outer marker.
Full Definition
A low-power non-directional beacon (NDB) installed at the outer marker position of an instrument landing system (ILS) approach. It transmits a continuous radio signal that the aircraft's automatic direction finder (ADF) can home in on, allowing the pilot to navigate to the and the outer marker .