Definition
To depart from the assigned or planned flight path, usually temporarily, in order to avoid hazardous weather, traffic, terrain, or other obstacles before returning to the original route or being assigned a new clearance.
Plain English
To leave your planned route for a short while — usually to go around bad weather or another problem — and then either rejoin the route or follow a new one given by ATC.
Context Anchor
Seen in instrument flying when a pilot needs to avoid thunderstorms, heavy precipitation, or other unsafe weather along the planned route.
Derivation
From Latin deviare, meaning 'to turn off the road' (de- 'away from' + via 'way, road'). The aviation use keeps the original sense exactly: turning away from the path you were on.
Why Pilots Care
Uncontrolled deviation risks loss of separation or navigation errors, while a properly coordinated deviation prevents encounters with thunderstorms or icing.
Intuition Check
Do not read this as accidentally wandering away from the course. In this context, deviate off course usually means a deliberate change away from the planned or assigned path, often for safety.
Example Sentence 1
The pilot asked ATC for permission to deviate off course twenty miles to the north to avoid a line of thunderstorms.
Example Sentence 2
ATC cleared the flight to deviate off course as needed for weather avoidance.