From Glossary of Meteorology
(Sometimes called grid system, G system.) A system of navigation by use of an arbitrary set of grid lines, as constructed to navigation by use of latitude and longitude.
A grid navigation chart contains a set of straight lines (grid navigations) parallel to the 180° and 0° meridians. The lines are used like true meridians on a Mercator map. With respect to the grid meridians, great circles and rhumb lines are identical. Thus, some navigational problems are greatly simplified when grid north is used instead of true north.