The Collaborative International Dictionary of English v.0.48:

Melanin \Mel"a*nin\, n. [Gr. me`las, me`lanos, black.]
   (Physiol.)
   A black pigment found in the pigment-bearing cells of the
   skin (particularly in the skin of the negro), in the
   epithelial cells of the external layer of the retina (then
   called fuscin), in the outer layer of the choroid, and
   elsewhere. It is supposed to be derived from the
   decomposition of hemoglobin.
   [1913 Webster]

WordNet (r) 3.0 (2006):

melanin
    n 1: insoluble pigments that account for the color of e.g. skin
         and scales and feathers