[summary: The stabilizer of an element $~$x$~$ under the action of a group $~$G$~$ is the set (actually a subgroup) of elements of $~$G$~$ which leave $~$x$~$ unchanged. ]
Let the Group $~$G$~$ act on the set $~$X$~$. Then for each element $~$x \in X$~$, the stabiliser of $~$x$~$ under $~$G$~$ is $~$\mathrm{Stab}_G(x) = \{ g \in G: g(x) = x \}$~$. That is, it is the collection of elements of $~$G$~$ which do not move $~$x$~$ under the action.
The stabiliser of $~$x$~$ is a subgroup of $~$G$~$, for any $~$x \in X$~$. (Proof.)
A closely related notion is that of the orbit of $~$x$~$, and the very important Orbit-Stabiliser theorem linking the two.