Definition
Turns made by reference to the magnetic compass alone, using calculated lead or lag values to roll out on a desired heading. Because the magnetic compass is affected by acceleration and turning errors (most pronounced on northerly and southerly headings in the Northern Hemisphere), the pilot must under-shoot or over-shoot the target heading by a specific number of degrees to compensate.
Plain English
Turning to a new heading using only the magnetic compass, while accounting for the fact that the compass briefly lies during turns. The pilot stops the turn early or late on purpose so the compass settles on the intended heading.
Context Anchor
Seen in instrument flying, especially when discussing turns using the magnetic compass instead of a heading indicator.
Derivation
Compass comes from older words meaning to measure or go around. That fits aviation because the magnetic compass measures direction around the full circle of headings.
Why Pilots Care
Allows continued accurate heading control when the primary heading indicator fails.
Grounding Statement
During a compass turn, the aircraft is changing direction faster and more smoothly than the compass card can always show accurately.
Intuition Check
Do not assume the compass shows your exact heading during the turn. In a compass turn, the compass is useful, but its indication must be corrected for turning error.
Example Sentence 1
With the heading indicator failed, the pilot used compass turns to come around to a heading of 360, rolling out early to compensate for northerly turning error.
Example Sentence 2
With the directional gyro failed, the crew maintained headings using compass turns throughout the approach.