Plain English Meaning
The direction that points the opposite way from another direction. If one direction is 090 degrees (east), its reciprocal is 270 degrees (west).
Full Definition
The bearing that lies in the exact opposite direction from a given bearing, found by adding or subtracting 180 degrees. In ADF navigation, it is the bearing from the aircraft to the station when you know the bearing from the station to the aircraft, or vice versa.
Context Anchor
Seen in ADF navigation when a pilot is figuring out the aircraft’s position in relation to a radio station.
Derivation
Reciprocal comes from the Latin reciprocus, meaning 'moving back and forth' or 'returning the same way.' That captures the idea well: a reciprocal bearing is the bearing that goes back the way you came.
Why Pilots Care
Quickly gives the heading needed to fly back along the same line or return to a station without additional calculations.