Source : Webster's Revised Unabridged Dictionary (1913)

Methaemoglobin \Met`h[ae]m*o*glo"bin\ (? or ?), n. [Pref. met- +
   h[ae]moglobin.] (Physiol. Chem.)
   A stable crystalline compound obtained by the decomposition
   of hemoglobin. It is found in old blood stains.