From AMS Glossary
A coordinate used in geomagnetism bearing the same relation to the geomagnetic dipole equator as geographic latitude does to the geographic equator.
When meridional variations in phenomena closely related to the earth's magnetic field are plotted according to geomagnetic rather than geographic latitude, more clear-cut relations are usually depicted. Cosmic ray intensities and auroral frequencies are examples of quantities best studied as functions of this type of latitude.
Campbell, W. H. 1997. Introduction to Geomagnetic Fields. Cambridge Univ. Press, . p. 32.