Linked from
The 40 pages that link to Group action, each with the reason it gives.
Conjugacy classNarrower topic: Conjugation is an action of a group on itself.
Burnside's lemmaNarrower topic: The lemma applies to points moved by a group action.
Geometric group theoryNarrower topic: It is the basic way a group acts on a space.
Cheryl PraegerNarrower topic: Permutation groups arise precisely when a group acts on a set.