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Meaning of rotation-inversion axis

ro•ta'tion-in•ver'sion ax"is

Pronunciation: (rō-tā'shun in-vûr'zhun, -shun), [key]
— Crystall. Crystall.
  1. 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 calledCf.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.