The Collaborative International Dictionary of English v.0.48:

hemosiderin \hemosiderin\ n. (Med.)
   a granular yellowish-brown substance composed of protein and
   ferric oxide, resulting from the breakdown of hemoglobin; it
   has a higher iron content than ferritin, and its presence in
   body tissues or phagocytes can be a symptom of disturbed iron
   metabolism.
   [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

hemosiderin
    n 1: a granular brown substance composed of ferric oxide; left
         from the breakdown of hemoglobin; can be a sign of
         disturbed iron metabolism [syn: hemosiderin,
         haemosiderin]