1. An unpaired anatomical part; such as, the azygos vein.
2. Occurring singly, not in pairs.

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).