Abscissa, Ordinate and ?? for z-axis?
Solution 1:
It apparently is called "applicate". See http://en.wikipedia.org/wiki/Cartesian_coordinate_system for reference.
For 3D diagrams, the names "abscissa" and "ordinate" are rarely used for x and y, respectively. When they are, the z-coordinate is sometimes called the applicate. The words abscissa, ordinate and applicate are sometimes used to refer to coordinate axes rather than the coordinate values.
Which again refers to https://www.encyclopediaofmath.org/index.php/Cartesian_orthogonal_coordinate_system .