Source : Webster's Revised Unabridged Dictionary (1913)

Caffeine \Caf*fe"ine\, n. [Cf. F. caf['e]ine. See {Coffee}.]
   (Chem.)
   A white, bitter, crystallizable substance, obtained from
   coffee. It is identical with the alkaloid {theine} from tea
   leaves, and with {guaranine} from guarana.

Source : WordNet®

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