a brownish, crystalline substance containing ferric iron, formed in the blood by the oxidation of hemoglobin, as by the action of certain drugs or in the decomposition of the blood, and no longer able to combine reversibly with oxygen
See methemoglobin in American Heritage Dictionary 4
(mĕt-hēˈmə-glōˌbĭn)
noun
A brownish-red form of hemoglobin that occurs when hemoglobin is oxidized either during decomposition of the blood or by the action of various oxidizing drugs or toxic agents. It contains iron in the ferric state and cannot function as an oxygen carrier.