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.
Plain English
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.
Context Anchor
Seen in magnetic compass error discussions, navigation charts, and course planning where true directions must be related to compass directions.
Derivation
From the Latin declinare, meaning 'to lean away' or 'to turn aside.' The compass needle 'leans away' from true north by this angle. The same word is used in astronomy for how far a star sits north or south of the celestial equator -- the common idea is angular offset from a reference line.
Why Pilots Care
Failing to apply declination causes heading errors that grow over distance.
Analogy
Think of two arrows drawn from the same point: one points to the top of the map, and the other points where a compass says north is. Declination is the angle between those two arrows.
Intuition Check
Declination does not mean that something is decreasing or getting worse here. It means an angular offset: magnetic north is turned a certain number of degrees away from true north at that location.
Example Sentence 1
The chart showed an easterly declination of 8 degrees, so the pilot adjusted the true course before setting the heading bug.
Example Sentence 2
Sectional charts list declination values that pilots apply before flight to keep navigation accurate.