The Collaborative International Dictionary of English v.0.48:

Melanosis \Mel`a*no"sis\, [NL., fr. Gr. ? a growing black, fr.
   me`las, me`lanos, black.] (Med.)
   The morbid deposition of black matter, often of a malignant
   character, causing pigmented tumors.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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