The Collaborative International Dictionary of English v.0.48:

Melanism \Mel"a*nism\ (m[e^]l"[.a]*n[i^]z'm), n. [Gr. me`las,
   -anos, black.]
   [1913 Webster]
   1. An undue development of dark-colored pigment in the skin
      or its appendages; -- the opposite of albinism.
      [1913 Webster]

   2. (Med.) A disease; black jaundice. See Melaena.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Melanism \Mel"a*nism\ (m[e^]l"[.a]*n[i^]z'm), n. (Ethnol.)
   The character of having a high degree of pigmentation, as
   shown in dark skin, eyes, and hair.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

melanism
    n 1: a condition characterized by abnormal deposits of melanin
         (especially in the skin) [syn: melanosis, melanism]