Definition of "z-direction" []

• The direction aligned with the z-axis of a coordinate system. (noun)

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• "Similarly, observer O sees an airplane moving at velocity in the x-direction, in the y-direction, and in the z-direction."
• "Let the system consist of a pair of photons 1 and 2 propagating in the z-direction."
• "Only during the R-measurement, the spin and the position of the R-particle get correlated and the R-particle's trajectory along the z-direction is dictated by the value of its (intrinsic) z-spin."