• In a two-dimensional Cartesian coordinate system, the horizontal axis, the other being the y-axis. Also, one of the three mutually-perpendicular axes in a three-dimensional Cartesian coordinate system, the others being the y- and z-axes.
  • One of three reference axes in a quartz crystal, the others being the y- and z- axes.
  • In a CRT, the axis representing the horizontal deflection of the electron beam.

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