In anatomy, azygos, from the Greek άζυξ, refers to a structure that is unpaired. This is relatively unusual, as most elements of anatomy reflect bilateral symmetry. Another term for the same concept is impar.

Well-known structures incorporating this nomenclature include the azygos vein

Azygos vein

The azygos vein is a vein running up the right side of the thoracic vertebral column. It can also provide an alternate path for blood to the right atrium by allowing the blood to flow between the venae cavae when one vena cava is blocked.-Structure:...

, ganglion impar

Ganglion impar

The pelvic portion of each sympathetic trunk is situated in front of the sacrum, medial to the anterior sacral foramina. It consists of four or five small sacral ganglia, connected together by interganglionic cords, and continuous above with the abdominal portion...

, and the anatomical variant known as the azygos lobe

Azygos lobe

In human anatomy, an azygos lobe is a rare congenital variation of the upper lobe of the right lung. Embryologically, it arises from an anomalous lateral course of the azygos vein in a pleural septum within the upper lobe...

of the lung.