地磁偏角是指地球上任一处的磁北方向和正北方向之间的夹角。当地磁北向实际偏东时，地磁偏角为正，反之为负。

地磁偏角在历史上最早由中国北宋科学家沈括发现并记录在著作《梦溪笔谈》中：「方家以磁石磨针锋，则能指南，然常微偏东，不全南也。」而西方最早的记录则在此之后约400年。

Magnetic declination or variation is the angle on the horizontal plane between magnetic north (the direction the north end of a compass needle points, corresponding to the direction of the Earth's magnetic field lines) and true north (the direction along a meridian towards the geographic North Pole). This angle varies depending on position on the Earth's surface, and changes over time.