Meaning of screw axis

screw' ax"is

Pronunciation: [key]
— Crystall. Crystall.
  1. a symmetry element of a space group such that a rotation of the lattice about the axis and a translation of the lattice some fraction of the lattice's unit distance brings the lattice back to its original position.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.