From AMS Glossary
- (Also called variation.) In terrestrial magnetism, at any given location, the angle between the geographic meridian and the magnetic meridian; that is, the angle between true north and magnetic north.
Declination is either "east" or "west" according as the compass needle points to the east or west of the geographic meridian. Lines of constant declination are called isogonic lines and that of zero declination is called the agonic line.
- In astronomy, the angular distance between any given celestial body and the celestial equator, measured along a great circle (hour circle) passing through the celestial poles; thus, the astronomical analog of geographic latitude.
Declination is positive for positions north of the equator and negative for positions south of the equator. Declination and right ascension are the coordinates used in positional astronomy.