An element $a$ of a ring (with a unit element 1) for which there exists an inverse, i.e. an element $b$ such that $ab=ba=1$. In the theory of algebraic numbers and algebraic functions such elements are also called units.
The phrases divisor of unity or invertible element are also used for this notion.
See Divisibility in rings for the general theory.
Unit divisor. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Unit_divisor&oldid=35236