Definition:Normal Subset/Definition 1

Definition
Let $\left({G, \circ}\right)$ be a group.

Let $S \subseteq G$ be a general subset of $G$.

Then $S$ is a normal subset of $G$ iff:
 * $\forall g \in G: g \circ S = S \circ g$

Also see

 * Equivalence of Definitions of Normal Subset