magnetic variation

1. Differences in the earth's magnetic field in time and location.
2. See magnetic declination.

(DOD, NATO) 1. In navigation, at a given place and time, the horizontal angle between the true north and magnetic north measured east or west according to whether magnetic north lies east or west of true north. See also magnetic declination. 2. In cartography, the annual change in direction of the horizontal component of the Earth's magnetic field.

The noun has one meaning:

Meaning #1: the angle (at a particular location) between magnetic north and true north
  Synonyms: magnetic declination, variation