Plain English Meaning
A small radio beacon placed at the outer marker of an ILS approach. Pilots can home in on its signal to help line up with the runway and to confirm they are passing the outer marker.
Full Definition
A low-power, low or medium frequency non-directional radio beacon installed at the site of the outer marker of an Instrument Landing System (ILS), used by pilots as a navigation aid for tracking inbound to the final approach course and for identifying the outer marker position.
Context Anchor
Seen on instrument approach charts and in instrument approach discussions, especially with that use a near the outer marker.