- noun the horizontal axis of a graph
- 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.