Definition:Invertible Element

Definition
Let $\left({S, \circ}\right)$ be an algebraic structure which has an identity $e_S$.

If $x \in S$ has an inverse, then $x$ is said to be invertible for $\circ$.

That is, $x$ is invertible :


 * $\exists y \in S: x \circ y = e_S = y \circ x$

Also see
In the context of a ring $\left({R, +, \circ}\right)$, an element that is invertible in the semigroup $\left({R, \circ}\right)$ is called a unit of $\left({R, +, \circ}\right)$.