The Collaborative International Dictionary of English v.0.48:

Creatinin \Cre*at"i*nin\ (kr?-?t"?-n?n), n. (Physiol. Chem.)
   A white, crystalline, nitrogenous body closely related to
   creatin but more basic in its properties, formed from the
   latter by the action of acids, and occurring naturally in
   muscle tissue and in urine. [Written also kretinine.]
   [1913 Webster]