Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

agonic line
     n : an imaginary line connecting points on the Earth's surface
         where the magnetic declination is zero