rotation-inversion axis

[ roh-tey-shuh n in-vur-zhuh n, -shuh n ]
/ roʊˈteɪ ʃən ɪnˈvɜr ʒən, -ʃən /

noun Crystallography.

an axis of a crystal such that rotating about the axis and then inverting the crystal brings the crystal back to its original position.
Also called symmetry axis of rotary inversion.
Compare symmetry element.