The Collaborative International Dictionary of English v.0.48:

Quinine \Qui"nine\, n. [F. (cf. Sp. quinina), fr. Sp. quina, or
   quinaquina, Peruvian bark, fr. Peruv. kina, quina, bark. Cf.
   Kinic.] (Chem.)
   An alkaloid extracted from the bark of several species of
   cinchona (esp. Cinchona Calisaya) as a bitter white
   crystalline substance, C20H24N2O2. Hence, by extension
   (Med.), any of the salts of this alkaloid, as the acetate,
   chloride, sulphate, etc., employed as a febrifuge or
   antiperiodic. Called also quinia, quinina, etc. [Written
   also chinine.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

quinine
    n 1: a bitter alkaloid extracted from chinchona bark; used in
         malaria therapy