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From 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]