Molecular symmetry
Rotation-reflection axis: an axis around which a  rotation by , followed by a reflection in a plane perpendicular to it, leaves the molecule unchanged. Also called  an n-fold improper rotation axis, it is shortened to Sn, with n necessarily even. Examples are present in tetrahedral silicon tetrafluoride, with three S4 axes, and the staggered conformation of ethane with one S6 axis.