The Collaborative International Dictionary
Agonic line
Agonic \A*gon"ic\, a. [Gr. ? without angles; 'a priv. + ? an angle.] Not forming an angle.
Agonic line (Physics), an imaginary line on the earth's surface passing through those places where the magnetic needle points to the true north; the line of no magnetic variation. There is one such line in the Western hemisphere, and another in the Eastern hemisphere.
Wiktionary
agonic line
n. (context cartography navigation English) A line on a chart or map joining points of zero magnetic declination.
WordNet
agonic line
n. an imaginary line connecting points on the Earth's surface where the magnetic declination is zero