The Collaborative International Dictionary of English v.0.48:

magnetic declination \magnetic declination\ n.
   The angle (at a particular location) between magnetic north
   and true north; called also magnetic variation.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

magnetic declination
    n 1: the angle (at a particular location) between magnetic north
         and true north [syn: magnetic declination, magnetic
         variation, variation]