Melanism is a condition in which a bodily part is morphologically dark due to the unusually high deposition of melanin. Melanin is a dark pigment produced by the specialized cells called melanocytes. This pigment attributes to the dark coloration of the hair, eyes, skin, plumage, pelage, and other bodily parts of a living organism. A black panther common in the equatorial rainforest of Malaya and the tropical rainforest of Mount Kenya is an example of an animal with melanism.
Melanism is in contrast to albinism. The latter is a condition where melanin production is lacking or insufficient. In humans, albinism results in reddish eyes, whitish hair and very pale skin.
In some literature, melanism is used synonymously with melanosis since both of these conditions result in dark pigmentation. However, melanosis is the morbid deposition of the black pigment and is often associated with malignancy resulting further into pigmented tumors.1
1 Webster's Revised Unabridged Dictionary (1913) Melanosis. C. & G. Merriam Co. Springfield, Massachusetts.