Plain English Meaning
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.
Full 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.
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.