X, Y, Z — horizontal, vertical and ...?

Solution 1:

I doubt there is such a co-hyponym (if we can call it that) to horizontal and vertical. You'll need to use an alternative name.

If you imagine the 3 axes, then the Z would appear "on the same level" as the X one. Depending on which ones you consider, 2 of the 3 will appear as such and actually, they are.

If you look at the Wikipedia page for Cartesian Coordinate System, under the section Cartesian Space it says:

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.

Emphasis mine. It says they are rarely used, but I doubt there are many other alternative terms, other than Z-axis, depth, and so on; they're the most appropriate terms, if you're looking for something technical.

Solution 2:

In aviation we use the terms longitudinal, lateral and normal (or vertical) for the three axes. See this description.

Note that these are fixed relative to the aircraft, not the earth.

Solution 3:

Original

Perhaps it's time to coin a new term? Here are a few possibilities I came up with:

• Applicatal (derived from applicate)
• Depthical (derived from depth)
• Zedical (derived from Z)
• Fordinal (derived from forward)

Edit

Upon further research, it appears that in the realm of print media, they refer to the 3rd axis of linearity as "stacked". So you have horizontal, vertical, and stacked printing layouts. Here is a link to the best explanation I could find:

• http://www.elkriversystems.com/PrintedOutput.aspx

In hind sight, when making user interface layouts where the items move along the Z-axis (in a list), I have referred to them as being stacked. Given that this is in the context of programming, stacked may work for you if you're referencing the linearity of a layout.