adjective
Definition: Referring to something that is single or unpaired, not part of a pair.
Example: The azygos vein is an important structure in the human body that is not paired with another vein.
noun
Definition: In anatomy, a structure that is not paired with another similar structure, which is uncommon since most anatomical features are symmetrical.
Example: The azygos vein plays an important role in the circulatory system by draining blood from the thoracic wall.