\begin{definition}[Definition:Invertible Fractional Ideal]
Let $R$ be an integral domain with fraction field $K$.
Let $I\subseteq K$ be a fractional ideal of $R$.
Then $I$ is '''invertible''' {{iff}} there exists a fractional ideal $J\subseteq K$ such that their product is the unit ideal of $R$:
:$I J = \ideal 1$
\end{definition}