Plain English Meaning

It is the angle between the direction your compass calls 'north' and the actual top of the Earth. The two are not in the same place, so the compass is always off from true north by some amount, and that amount is called declination.

Full Definition

Declination is another name for magnetic variation: the angular difference, measured in degrees east or west, between true north (the geographic North Pole) and magnetic north (the point the compass needle actually points to) at a given location on the Earth's surface.

Context Anchor

Seen in magnetic compass error discussions, navigation charts, and course planning where true directions must be related to compass directions.