Plain English Meaning

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.

Full 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 (the between true and ).