Agonic Line Definition

ā-gŏnĭk, ə-

noun

An imaginary line on the earth's surface connecting points where the magnetic declination is zero.

American Heritage

An imaginary line on the earth's surface along which true north and magnetic north are identical: at any point along this line, the declination of a compass needle equals zero.

Webster's New World

(cartography, navigation) A line on a chart or map joining points of zero magnetic declination.

Wiktionary

Other Word Forms of Agonic Line

Noun

Singular:

agonic line

Plural:

agonic lines

Origin of Agonic Line

Greek agōnos having no angle (a- without a–¹) (gōniā angle diagonal) –ic

From American Heritage Dictionary of the English Language, 5th Edition
agonic + line

From Wiktionary