The Collaborative International Dictionary of English v.0.48:

Vicinal \Vic"i*nal\ (?; 277), a. [L. vicinalis: cf. F. vicinal.]
   1. Near; vicine. --T. Warton.
      [1913 Webster]

   2. (Organic Chem.) Having the substituted groups on the same
      carbon atom.
      [PJC]

   Vicinal planes (Min.), subordinate planes on a crystal,
      which are very near to the fundamental planes in angles,
      and sometimes take their place. They have in general very
      complex symbols.
      [1913 Webster]