Plain English Meaning

The compass does not point to the actual top of the Earth. It points to a slightly different spot called magnetic north. Magnetic variation is the gap, in degrees, between those two points as seen from where you are flying.

Full Definition

The angular difference between true north (the geographic North Pole) and magnetic north (where a compass needle points) at a given location on the Earth's surface. Magnetic variation is expressed in degrees east or west and changes depending on geographic location. It must be applied when converting between true and magnetic headings or courses.

Context Anchor

Seen on aviation charts, instrument procedures, and navigation data when courses are shown or converted between true and magnetic references.