Unipotent automorphism

Definition with symbols (right action convention)
Let $$G$$ be a group. For $$\sigma \in \operatorname{Aut}(G)$$ and $$g \in G$$, denote by $$[g,\sigma]$$ the element $$g^{-1}\sigma(g)$$. Then $$\sigma$$ is said to be unipotent of class $$n$$ if for any $$g \in G$$:

$$[[[\ldots[g,\sigma],\sigma],\ldots] = e$$

with $$\sigma$$ written $$n$$ times.