Magnetic Deviation Meaning

Example: Used other than as an idiom: see magnetic,‎ deviation.
1904, Ernest Rutherford, Radio-activity, Dover, 2004, page 117,
  Preliminary experiments with radium of activity 1000 showed that it was very difficult to determine the magnetic deviation of the α rays.
Magnetic declination.
1998, John R. Ellis, Objectifying Real-Time Systems, page 483,
  This angular difference is the local magnetic deviation (Figure C.2). Navigation systems normally work relative to True North and therefore must remove the local magnetic deviation.