The axis in three-dimensional Cartesian coordinates which is usually oriented vertically. Cylindrical
coordinates are defined such that the 
-axis is the axis about which the azimuthal coordinate 
is measured.
z-Axis
See also
Axis, x-Axis, y-AxisExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "z-Axis." From MathWorld--A Wolfram Web Resource. https://wwn.ikan3.com/7hz2929k26_9nqeuhigsnjcgsjpnuemse/z-Axis.html