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.