Definition
A marker beacon at or near the glideslope intercept altitude of an ILS approach, normally located four to seven miles from the runway threshold. It transmits a 75 MHz signal upward in a narrow fan-shaped pattern, identified in the cockpit by a series of low-pitched dashes (two per second) and a flashing blue marker light. It indicates the position at which an aircraft on the localizer course should intercept the glideslope and begin the final descent.
Plain English
A radio beacon on the ground, several miles from the runway, that tells the pilot 'you are now at the point where you start descending toward the runway on the ILS.' It signals you with a tone in the headset and a blue light on the panel.
Context Anchor
You may see Outer Marker shown on an instrument approach chart and receive its indication in the cockpit during an ILS approach.
Derivation
Called 'outer' because it is the marker farthest from the runway in the ILS marker sequence (outer, middle, and sometimes inner). 'Marker' because it marks a specific position along the approach path.
Why Pilots Care
It confirms the aircraft is at the proper point to intercept the glideslope and begin a stabilized descent to the runway.
Analogy
It is like a mile marker on a road, but for an aircraft approaching a runway: passing it confirms where you are along the route.
Intuition Check
Outer Marker does not mean a painted sign or physical marker outside the airport. In this context, it means a radio signal point that marks a position on an instrument approach.
Example Sentence 1
Crossing the outer marker, the pilot verified the altitude on the glideslope and continued the descent toward the runway.
Example Sentence 2
They crossed the outer marker at the published altitude shown on the approach chart.