Definition
The compass heading or bearing that lies exactly 180° opposite a given heading or bearing. The reciprocal of any heading is found by adding or subtracting 180°.
Plain English
The direction that points the opposite way from where you are heading or looking. If you are heading one way, the reciprocal is the heading that takes you straight back.
Context Anchor
Seen when setting or interpreting a course on a Horizontal Situation Indicator, and when comparing opposite runway directions or return courses.
Derivation
From Latin reciprocus, meaning 'moving backward and forward' or 'returning the same way.' That sense fits aviation use directly: the reciprocal heading is the one that would take you back the way you came.
Why Pilots Care
Confusing a heading with its reciprocal is a common navigation error. Flying the reciprocal of an intended course takes you 180° away from where you wanted to go. It also matters when intercepting a radial inbound versus outbound, and when reading the tail of an HSI needle instead of the head.
Analogy
On a compass, the reciprocal is the number directly across the circle from the one you are using, like 090 degrees and 270 degrees.
Intuition Check
Reciprocal does not just mean related or exchanged here. In this aviation use, it means the exact opposite direction on the compass: 180 degrees away.
Example Sentence 1
After overflying the station, the pilot turned to the reciprocal heading of 270° to track outbound on the 090° radial.
Example Sentence 2
The HSI compass rose displays the reciprocal heading directly opposite the current course pointer.