Abstract Algebra
《Abstract Algebra》 · 第 134 页 · Section 4.4「Automorphisms」，Definition
A subgroup $H$ of a group $G$ is called characteristic in $G$, denoted $H\operatorname{char}G$, if every automorphism of $G$ maps $H$ to itself, i.e., $\sigma(H)=H$ for all $\sigma\in\operatorname{Aut}(G)$.