\begin{definition}[Definition:Group of Outer Automorphisms]
Let $G$ be a group.
Let $\Aut G$ be the automorphism group of $G$.
Let $\Inn G$ be the inner automorphism group of $G$.
Let $\dfrac {\Aut G} {\Inn G}$ be the quotient group of $\Aut G$ by $\Inn G$.
Then $\dfrac {\Aut G} {\Inn G}$ is called the '''group of outer automorphisms''' of $G$.
This group $\dfrac {\Aut G} {\Inn G}$ is often denoted $\Out G$.
\end{definition}