Oxyhemoglobin: The oxygen-loaded form of hemoglobin, the predominant protein in red blood cells.
Hemoglobin is a protein molecule that binds to oxygen. Hemoglobin forms an unstable, reversible bond with oxygen. In its oxygen-loaded form, it is called oxyhemoglobin and is bright red. In the oxygen-unloaded form it is called deoxyhemoglobin and is purple-blue.