Stabilizer

of an element $a$ in a set $M$

The subgroup $G_a$ of a group of transformations $G$, operating on a set $M$, (cf. Group action) consisting of the transformations that leave the element $a$ fixed: $G_a = \{ g \in G : ag = a \}$. The stabilizer of $a$ is also called the isotropy group of $a$, the isotropy subgroup of $a$ or the stationary subgroup of $a$. If $b \in M$ is in the orbit of $a$, so $b = af$ with $f \in G$, then $G_b = f^{-1}G_af$. If one considers the action of the group $G$ on itself by conjugation, the stabilizer of the element $g \in G$ will be the centralizer of this element in $G$; if the group acts by conjugation on the set of its subgroups, then the stabilizer of a subgroup $H$ will be the normalizer of this subgroup (cf. Normalizer of a subset).

Comments

In case $M$ is a set of mathematical structures, for instance a set of lattices in $\mathbf{R}^n$, on which a group $G$ acts, for instance the group of Euclidean motions, then the isotropy subgroup $G_m$ of $m \in M$ is the symmetry group of the structure $m \in M$.

References

[a1]	L. Michel, "Simple mathematical models for symmetry breaking" K. Maurin (ed.) R. Raczka (ed.) , Mathematical Physics and Physical Mathematics , Reidel (1976) pp. 251–262
[a2]	S. Helgason, "Differential geometry, Lie groups, and symmetric spaces" , Acad. Press (1978) pp. 121
[a3]	T. Petrie, J.D. Randall, "Transformation groups on manifolds" , M. Dekker (1984) pp. 8, 9