Definition
A departure from an assigned route, altitude, clearance, or planned course of action, either authorized by ATC or required by the pilot in response to weather, traffic, terrain, or an emergency. In a separate technical sense, deviation also refers to the magnetic compass error caused by metallic and electrical influences within the aircraft itself, which differs from variation (the difference between true and magnetic north).
Plain English
Deviation means moving off the path or instruction you were given. In flying, that can mean leaving an assigned route or altitude when you need to, or it can mean the small error in your compass caused by the airplane's own metal and wiring.
Context Anchor
Seen when using the magnetic compass, checking the compass correction card, or discussing basic navigation.
Derivation
From the Latin deviare, meaning "to turn off the road" (de- "away from" + via "way, road"). The aviation use carries that original sense directly: leaving the assigned path, whether that path is a cleared route or the direction the compass should be pointing.
Why Pilots Care
Uncorrected deviation produces heading errors that grow over distance and can lead to navigation mistakes, especially when relying on the magnetic compass.
Intuition Check
Deviation does not just mean any change from a plan here. In this context, it means compass error caused by the aircraft itself.
Example Sentence 1
The pilot requested a deviation twenty degrees right of course to avoid a line of thunderstorms.
Example Sentence 2
During the compass swing, deviation was recorded for each cardinal heading so the card could be updated.