Z-Axis
The third axis used to locate points in three-dimensional space.
The z-coordinate of a point is the third element of the ordered triplet of a point in three-dimensional space that represents its position on the third axis of a Cartesian coordinate system in space.
Example
In this illustration, the Cartesian coordinates of point P are (1, 3, 2). That is, its x-coordinate is 1, its y-coordinate is 3 and its z-coordinate is 2.
