Definition of "x-direction" []

• The direction aligned with the x-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."
• "Neither photon 1 nor photon 2 is in a definite polarization state when the pair is in the state | ψ, but their potentialities (in the terminology of Heisenberg 1958) are correlated: if by measurement or some other process the potentiality of photon 1 to be polarized along the x-direction or along the y-direction is actualized, then the same will be true of photon 2, and conversely."
• "Thus, if a quantum mechanical system (say, an electron) is described by a superposition of two given states, say, spin in x-direction equal +1/2 and spin in x-direction equal"