Wiktionary
neuromelanin
n. A dark pigment present in parts of the brain.
Wikipedia
Neuromelanin
Neuromelanin (NM) is a dark pigment found in the brain which is structurally related to melanin. It is a polymer of 5,6-dihydroxyindole monomers. Neuromelanin is expressed in large quantities in catecholaminergic cells of the substantia nigra pars compacta and locus coeruleus, giving dark color to the structures.