The Collaborative International Dictionary of English v.0.48:

Agonic \A*gon"ic\, a. [Gr. ? without angles; 'a priv. + ? an
   angle.]
   Not forming an angle.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

agonic line
    n 1: an imaginary line connecting points on the Earth's surface
         where the magnetic declination is zero