Fundamental domain

Subset of a space that contains exactly one point from each orbit of the action of a group

Given a topological space and a group acting on it, the images of a single point under the group action form an orbit of the action. A fundamental domain or fundamental region is a subset of the space which contains exactly one point from each of these orbits. It serves as a geometric realization for the abstract set of representatives of the orbits.

This keyword could refer to multiple things. Here are some suggestions: