Arboreal Galois representation
In arithmetic dynamics, an arboreal Galois representation is a continuous group homomorphism between the absolute Galois group of a field and the automorphism group of an infinite, regular, rooted tree. The study of arboreal Galois representations of goes back to the works of Odoni in 1980s.
Source: Wikipedia — Arboreal Galois representation (CC BY-SA 4.0)