The Collaborative International Dictionary of English v.0.48:

Hemoglobin \Hem"o*glo"bin\, n. [Hemo- + globe.] (Physiol.)
   The normal coloring matter of the red blood corpuscles of
   vertebrate animals. It is composed of hematin and globulin,
   and is also called h[ae]matoglobulin. In arterial blood, it
   is always combined with oxygen, and is then called
   oxyhemoglobin. It crystallizes under different forms from
   different animals, and when crystallized, is called
   h[ae]matocrystallin. See Blood crystal, under Blood.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hemoglobin
    n 1: a hemoprotein composed of globin and heme that gives red
         blood cells their characteristic color; function primarily
         to transport oxygen from the lungs to the body tissues;
         "fish have simpler hemoglobin than mammals" [syn:
         hemoglobin, haemoglobin, Hb]