ISOGONIC is a 8 letter word that starts with I.

1. \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.
   
   

2. \I`so*gon"ic\, a. (Zo["o]l.)
   Characterized by isogonism.