The Collaborative International Dictionary of English v.0.48:

caffein \caf*fe"in\, Caffeine \Caf*fe"ine\, n. [Cf. F.
   caf['e]ine. See Coffee.] (Chem.)
   A white, bitter, crystallizable substance, found in coffee
   and tea. It is identical with the alkaloid theine from tea
   leaves, and with guaranine from guarana. It is responsible
   for most of the stimulating effect of coffee or tea.
   [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

caffeine
    n 1: a bitter alkaloid found in coffee and tea that is
         responsible for their stimulating effects [syn: caffeine,
         caffein]