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