Definition
A 75 MHz radio beacon associated with an Instrument Landing System (ILS) approach, located on the final approach course typically four to seven miles from the runway threshold, that transmits a signal vertically to mark the point where an aircraft on the glideslope will normally intercept it at the published altitude. When an aircraft passes over it, the airborne marker beacon receiver activates a blue light and a low-pitched tone of continuous dashes (two per second at 400 Hz).
Plain English
A small radio transmitter on the ground that points its signal straight up. As your aircraft flies over it during an instrument approach, your cockpit gives you a blue light and a tone, telling you that you've reached a known fixed point on the approach -- usually where you should be starting your descent down the glideslope.
Context Anchor
Seen on instrument approach profile views, especially older instrument landing system approach charts, where it may be labeled OM along the final approach path.
Derivation
Called 'outer' because it's the outermost of the marker beacons on a traditional ILS (outer, middle, and sometimes inner). 'Marker' because it marks a specific point along the approach course.
Why Pilots Care
It gives a precise fix for starting final descent, timing the approach, and executing a missed approach if required.
Intuition Check
Do not read “outer” as simply outside the airport. Here, “outer” means the marker beacon farthest from the runway along the approach path.
Example Sentence 1
Crossing the outer marker, the pilot confirmed altitude on the glideslope and continued the descent toward the runway.
Example Sentence 2
The pilot noted the blue light and 400 Hz tone confirming passage over the outer marker.