The Collaborative International Dictionary of English v.0.48:

Isoclinal \I`so*cli"nal\, Isoclinic \I`so*clin"ic\, a. [Iso- +
   Gr. ? to incline.]
   Of or pertaining to, or indicating, equality of inclination
   or dip; having equal inclination or dip.
   [1913 Webster]

   Isoclinal lines (Magnetism), lines on the earth's surface
      connecting places at which a dipping needle indicates the
      same inclination or dip.
      [1913 Webster]

WordNet (r) 3.0 (2006):

isoclinal
    adj 1: having equal magnetic inclinations [syn: isoclinal,
           isoclinic]
    n 1: an isogram connecting points of equal magnetic inclination
         [syn: isoclinic line, isoclinal]