Definition - What does Declination mean?
Declination is the difference measured in angular degrees, i.e., the difference between the magnetic north and true north. The compass used by geologists’ points to the direction of the magnetic field's horizontal component from where the compass is located and never points to a direction of any single point. The geologists measure the difference between horizontal trace and the true north, which is measured in the angular degrees of the magnetic field for the location that allows them to correct their compass for magnetic field in the area.
Petropedia explains Declination
It is believed that a compass needle points to the magnetic north pole. However, this is not technically true, as the needle points to direction of horizontal component of the magnetic field. Declination is the angle between the true north and the direction at which the compass points. It varies from 0° to 30° in the most populated areas of the world. As the earth's plates shift, these values also slightly change over time. It is a term preferred by people studying magnetic field and is generally used by land navigators. The term magnetic declination is also used to refer to declination at times. It is used in explanatory information of maps for the wilderness or navigational use and the topographic quadrangle maps of the United States Geological Survey.