1. A reference to an anatomical part 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 azygous vein, which drains blood from the chest.

The physicians examined Richard'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".