Definition
The angular difference between true north (the geographic North Pole) and magnetic north (where a compass points) at a given location on the Earth's surface, expressed in degrees east or west.
Plain English
The Earth's geographic North Pole and the place a compass actually points to are not the same spot. Variation is the angle between those two directions at wherever you happen to be flying. It changes depending on where you are on the Earth.
Context Anchor
Seen when using charts, magnetic compasses, and instrument navigation where a direction must be changed between true direction and magnetic direction.
Derivation
From Latin variatio, meaning 'a change or difference.' The word captures exactly what is happening: the compass direction varies from the true direction by some amount, and that amount depends on where you are.
Why Pilots Care
Pilots apply variation to convert between true courses used for flight planning and magnetic headings flown with the compass.
Intuition Check
Variation does not mean a random change or a small error here. In this context, it means the known local angle between true north and magnetic north, labeled east or west.
Example Sentence 1
Before departure, the pilot checked the isogonic line on the sectional chart and applied 8 degrees west variation to convert the true course into a magnetic course.
Example Sentence 2
Sectional charts mark variation values so pilots can adjust headings before flight.