The Collaborative International Dictionary of English v.0.48:

Guanidine \Gua"ni*dine\, n. (Physiol. Chem.)
   A strongly alkaline base, NH2.CNH.NH2, formed by the
   oxidation of guanin, and also obtained combined with methyl
   in the decomposition of creatin. Boiled with dilute sulphuric
   acid, it yields urea and ammonia.
   [1913 Webster]