Geom. in a three-dimensional Cartesian coordinate system, the axis that is perpendicular to the x-axis and the y-axis and that is used to measure or plot the values of z
See z-axis in American Heritage Dictionary 4
(zēˈăkˌsĭs)
nounpl.z-ax·es(zēˈăkˌsēz)
One of three axes in a three-dimensional Cartesian coordinate system.