Definition
A cockpit instrument that combines a rotating magnetic compass card with one or two bearing pointers driven by navigation receivers (typically ADF and/or VOR). The compass card automatically aligns with magnetic north, and the pointers continuously indicate the magnetic bearing from the aircraft to the tuned station.
Plain English
An instrument that shows you which direction you are heading and, at the same time, points an arrow toward the radio station you have tuned in. Wherever the arrow points is the direction to that station from your aircraft.
Context Anchor
Seen in instrument flying when using panel instruments to maintain awareness of direction and the location of a radio navigation source.
Derivation
"Radio" because the pointers are driven by radio navigation receivers, and "magnetic" because the compass card underneath is referenced to magnetic north. The name describes exactly what the instrument shows: radio bearings displayed against a magnetic heading.
Why Pilots Care
Gives immediate, heading-adjusted bearing information so the pilot can turn directly toward or away from a station without manual calculations.
Intuition Check
Magnetic does not mean the instrument is simply a magnet. Here it means the directions shown are measured from magnetic north.
Example Sentence 1
After tuning the NDB, the pilot watched the RMI needle swing around and settle on the bearing to the station.
Example Sentence 2
With the RMI needle steady on 090 degrees, the aircraft was tracking directly toward the beacon.