a·zy·gos 
(ā-zīgəs) Anatomy
(ā-zīgəs) Anatomy adj. also a·zy·gous (-gəs)
Not one of a pair, as a vein or muscle; occurring singly.
n.
An unpaired anatomical structure.
[Greek : a-, a- + -zugos, yoked (from zugon, yoke, pair; see yeug- in the Appendix of Indo-European roots).]