\begin{definition}[Definition:Length of Group]
Let $G$ be a finite group.
The '''length''' of $G$ is the length of a composition series for $G$.
That is, the '''length''' of $G$ is the number of factors in a composition series for $G$ (not including $G$ itself).
The '''length''' of $G$ can be denoted $\map l G$.
By the Jordan-Hölder Theorem, all composition series for $G$ have the same length.
Therefore, the '''length''' of a finite group $G$ is well-defined.
\end{definition}