magnetic variation

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Differences in the earth's magnetic field in time and location.

from the GNU version of the Collaborative International Dictionary of English.

• noun Same as magnetic declination.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun military, nautical, navigation 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.
• noun cartography In cartography, the annual change in direction of the horizontal component of the Earth’s magnetic field.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the angle (at a particular location) between magnetic north and true north