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z axis[′zē ‚ak·səs]
The optical axis of a quartz crystal, perpendicular to both the x and y axes.
One of the three axes in a three-dimensional Cartesian coordinate system; in a rectangular coordinate system it is perpendicular to the x and y axes.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.