Definition:Length of Group

Definition
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.