Magnetic Declination is the bearing on a given date (reckoned east or west from the north branch of the celestial meridian plane) of magnetic north as determined by the positive pole of a freely suspended magnetic needle which is subject to no transient artificial disturbance.
An example provided by Compass Dude
Here is a map of the angles of declination for the U.S. as of 2010:
Notice the black line with 0� on it running down the Mississippi river? Along this particular line, both the geographic and magnetic north poles are in alignment so there is no declination. If you move East of this line, the magnetic north pole will pull your compass needle further and further to the West of geographic north - the angle of compass declination is West Declination. Moving west of the Mississippi river will pull your compass needle further and further to the East.
A few Videos about Magnetic Declination:
Related Term: Magnetic Variation
Regular or erratic change in magnetic declination. Not interchangeable with declination but old field notes use Var. as the declination of the compass.