Caffeine

from 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}]

from 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]