Definition of Vein

Vein: A blood vessel that carries blood low in oxygen content from the body back to the heart. The deoxygenated form of hemoglobin (deoxyhemoglobin) in venous blood makes it appear dark. Veins are part of the afferent wing of the circulatory system which returns blood to the heart.

By contrast, an artery is a vessel that carries blood high in oxygen away from the heart to the body. The oxygenated form of hemoglobin (oxyhemoglobin) in arterial blood makes it look bright red. Arteries are part of the efferent wing of the circulatory system which carries blood away from the heart.