Had to go back to the Book* for this one....as there has been too much stuff added since this one was.....a long time ago.
ISOGRIV - a line of a chart joining equal points of grivation
*Air Navigation Vol II - AF Manual 51-40 March 1955 (original copy)
Tinkicker
PS - Looks like Masif beat me to it by a few minutes.....congrats