\begin{definition}[Definition:Product Inverse]
Let $\struct {R, +, \circ}$ be a ring with unity.
Let $U_R$ denotes the group of units of $R$.
The inverse of $x \in U_R$ by $\circ$ is called the '''(ring) product inverse of $x$'''.
The usual means of denoting the product inverse of an element $x$ is by $x^{-1}$.
Thus it is distinguished from the additive inverse of $x$, that is, the (ring) negative of $x$, which is usually denoted $-x$.
\end{definition}