A symmetry consisting of rotation about an axis and translation parallel to that axis. A operation combines a half-turn with half a lattice period; applying it twice gives a complete lattice translation. It is a spatial symmetry, distinct from a screw dislocation.
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A screw axis is a concept in the field of crystallography and molecular symmetry that describes a particular type of symmetry operation. It refers to a combination of a rotation and a translation along the same axis. The screw axis is commonly denoted using a notation that combines a number (indicating the degree of rotation) and a fraction (indicating the translational component).