Plain English Meaning
A line on the map showing where your compass happens to point exactly to true north, with no east or west error to correct for.
Full Definition
An imaginary line on the Earth's surface connecting points where magnetic north and true north are aligned, so that magnetic variation is zero. A compass on the agonic line points to true north without any easterly or westerly variation correction.
Context Anchor
Seen in compass, navigation, and chart discussions when comparing magnetic direction with true direction.
Derivation
From Greek 'a-' meaning 'without' and 'gonia' meaning 'angle.' Literally 'no angle' — there is no angle between true north and magnetic north along this line. This contrasts with isogonic lines, which connect points of equal (but non-zero) variation.