Source : Webster's Revised Unabridged Dictionary (1913)

Orbital \Or"bit*al\, a.
   Of or pertaining to an orbit. ``Orbital revolution.'' --J. D.
   Forbes.

   {Orbital index} (Anat.), in the skull, the ratio of the
      vertical height to the transverse width of the orbit,
      which is taken as the standard, equal to 100.