Source : Webster's Revised Unabridged Dictionary (1913)

Isogonic \I`so*gon"ic\, a. [Iso- + Gr. ? angle.]
   Pertaining to, or noting, equal angles.

   {Isogonic lines} (Magnetism), lines traced on the surface of
      the globe, or upon a chart, connecting places at which the
      deviation of the magnetic needle from the meridian or true
      north is the same.