Definition:Self-Inverse Element

Definition
Let $\struct {S, \circ}$ be a monoid whose identity element is $e$.

Let $x \in S$ be an element of $S$.

Also known as
The definition is usually made in the context of a group.

Some sources refer to such an element as an involution.

Also see

 * Equivalence of Definitions of Self-Inverse
 * Inverse in Monoid is Unique