Variation

The angular difference between true and magnetic north at a location.

As taught in Direction: True, Magnetic, Wind and Compass Errors (Navigation and Sectional Charts) — a free lesson with cited FAA sources.

The angular difference between true north and magnetic north, shown by isogonic lines.

As taught in Building the Route: Checkpoints and Navigation Methods (Cross-Country Flight Planning) — a free lesson with cited FAA sources.