# Definition:Orbit (Group Theory)

*This page is about the orbit of a group action. For other uses, see Definition:Orbit.*

## Definition

Let $G$ be a group acting on a set $X$.

### Definition 1

The **orbit** of an element $x \in X$ is defined as:

- $\Orb x := \set {y \in X: \exists g \in G: y = g * x}$

where $*$ denotes the group action.

That is, $\Orb x = G * x$.

Thus the **orbit** of an element is all its possible destinations under the group action.

### Definition 2

Let $\mathcal R$ be the relation on $X$ defined as:

- $\forall x, y \in X: x \mathrel {\mathcal R} y \iff \exists g \in G: y = g * x$

where $*$ denotes the group action.

From Group Action Induces Equivalence Relation, $\mathcal R$ is an equivalence relation.

The **orbit** of $x$, denoted $\Orb x$, is the equivalence class of $x$ under $\mathcal R$.

## Length

The **length** of the orbit $\Orb x$ of $x$ is the number of elements of $X$ it contains:

- $\size {\Orb x}$

## Set of Orbits

The quotient set $X / \mathcal R_G$ is called the **set of orbits of $X$ under the action of $G$**.