1. Occurring singly; not occurring as one of a pair; such as, a vein or muscle: "The medical students learned that the superior vena cava (one of two very large veins) receives blood from the azygos vein, which drains blood from the chest."

"The physicians examined the patient's azygos veins to make sure that the three unpaired veins of the abdomen and thorax emptied properly into the inferior vena cave."

2. Etymology: from Greek meaning "unyoked" or "not a pair"; from a-, "without" + zygon, "a yoke".

One of a system of veins that drain the thoracic and abdominal walls which goes up from the right curving upward lumbar vein or the vena cava (large vein which returns blood to the heart), enters the thorax through the aortic orifice (largest artery) in the diaphragm, and terminates or ends in the superior vena cava (major blood vessel that returns blood from the head, neck, upper extremities and chest and feeds into the right atrium or chamber of the heart which pumps blood into the right ventricle for ejection into the pulmonary or lung circulation).