Definition
Unintended changes in the aircraft's heading away from the heading the pilot is trying to maintain during straight-and-level flight. In instrument flying, heading deviations are typically caused by failing to keep the wings level, allowing the aircraft to enter a shallow bank that produces a turn the pilot does not notice on the heading indicator until the heading has already drifted.
Plain English
The aircraft has wandered off the heading you were trying to hold. Instead of pointing where you wanted, the nose has drifted left or right of that direction.
Context Anchor
Seen in instrument flying when a pilot is trying to hold a steady heading during straight-and-level flight.
Derivation
Deviation comes from the Latin 'deviare,' meaning 'to turn off the road' (de- 'off' + via 'way'). A heading deviation is literally the aircraft turning off the path you intended.
Why Pilots Care
Unchecked heading deviations cause course errors, increase workload, and can lead to loss of situational awareness in instrument conditions.
Intuition Check
Do not read “heading deviation” as simply being blown sideways by wind. Heading is where the nose points; wind may make the airplane move over the ground differently from where the nose is pointed.
Example Sentence 1
During the cross-check, the pilot noticed a small heading deviation to the right and corrected it by leveling the wings and turning back to the assigned heading.
Example Sentence 2
During the IFR climb, heading deviations of more than five degrees triggered an immediate cross-check of the directional gyro.