Definition:Unit of Ring

Definition
A unit in a ring with unity $\left({R, +, \circ}\right)$ is an element of $R$ which is invertible under $\circ$ in $R$, i.e. that has an inverse.

The group of units of a ring $\left({R, +, \circ}\right)$ is often denoted $U_R$ (or just $U$ if there is no doubt what the ring is that's being talked about). Thus:


 * $x \in U_R \iff \exists y \in R: x \circ y = 1_R = y \circ x$

Also known as
Some sources use the term invertible element.

The notations $R^*$ and $R^\times$ are also seen for the set of units $U_R$.

Beware
Do not confuse a unit with the unity.