WordReference Random House Unabridged Dictionary of American English © 2020
agonic line, 
  • Geographyan imaginary line on the surface of the earth connecting all points at which the declination of the magnetic field of the earth is zero.
    • 1860–65

    Collins Concise English Dictionary © HarperCollins Publishers::
    agonic line n
    1. an imaginary line on the surface of the earth connecting points of zero magnetic declination
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