Definition
Small, deliberate corrections made to bank or heading when the aircraft has drifted off the desired flight path, applied in proportion to the size of the error and then promptly removed once the aircraft is back on course.
Plain English
Little corrections you make when the airplane has wandered off where you wanted it to be. The bigger the wander, the bigger the correction; once you're back on track, you stop correcting.
Context Anchor
Seen while using the attitude display and heading information during instrument flying, especially when holding a heading in straight-and-level flight or making small bank corrections.
Derivation
Deviation comes from the Latin 'deviare,' meaning 'to turn off the road' (de- 'off' + via 'way'). In flying, a deviation is anything that has taken the aircraft off its intended path, and the adjustment is the small input that puts it back.
Why Pilots Care
Prevents heading drift and overshoots that lead to course deviations or unstable approaches in IMC.
Intuition Check
Deviation adjustments do not mean compass-deviation calibration here. They mean small flying corrections made when an indication moves away from the desired value.
Example Sentence 1
When the heading drifted three degrees right of course, she made a small deviation adjustment with a shallow left bank and rolled wings level once back on heading.
Example Sentence 2
During the standard-rate turn, timely deviation adjustments kept the rollout exactly on the new heading.