1. The iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates (with the exception of the fish family Channichthyidae) as well as the tissues of some invertebrates. Hemoglobin in the blood carries oxygen from the respiratory organs (lungs or gills) to the rest of the body (i.e. the tissues) where it releases the oxygen to burn nutrients to provide energy to power the functions of the organism, and collects the resultant carbon dioxide to bring it back to the respiratory organs to be dispensed from the organism. (wikipedia.org)
2. The iron-containing substance in red blood cells that transports oxygen from the lungs to the rest of the body; it consists of a protein (globulin), and haem (a porphyrin ring with an atom of iron at its centre). (wiktionary.org)
4. 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. (wordnetweb.princeton.edu)
5. Four subunit globular oxygen carrying protein of the erythrocytes of vertebrates and some invertebrates.
It is a conjugated protein containing four haem groups and globin. There are two alpha and two beta chains (very similar to myoglobin) in adult humans, the haem moiety (an iron containing substituted porphyrin) is firmly held in a nonpolar crevice in each peptide chain. (biology-online.org)
Word origin: Shortening of hæmatoglobin (1845), from Greek haimato-, comb. form of haima (genitive haimatos) “blood” (see -emia) + globulin, a type of simple protein, from globule, formerly a word for “corpuscle of blood.”