The Collaborative International Dictionary of English v.0.48:

Xanthin \Xan"thin\, n. [Gr. xanqo`s yellow.]
   1. same as xanthine.
      [1913 Webster]

   2. (Chem.) A yellow insoluble coloring matter extracted from
      yellow flowers; specifically, the coloring matter of
      madder. [Formerly written also xanthein.]
      [1913 Webster]

   3. (Chem.) One of the gaseous or volatile decomposition
      products of the xanthates, and probably identical with
      carbon disulphide. [Obs.]
      [1913 Webster] Xanthine