grid variation

noun Navigation.
  1. the angle, at any point on the surface of the earth, between the magnetic and true meridians passing through that point.
Also called grivation.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018

British Dictionary definitions for grid variation

grid variation

noun
  1. nautical the angle between grid north and magnetic north at a point on a map or chartAlso called: grivation
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012