Why is there no depth in heightMapHeight?

I'm still struggling to learn how to deal with scripting terrain, how it holds it's heightmap as there is only 2 fields in the 2-dimensional array of the heightmap.

Why is there no depth field? There is only x & y, but I don't understand because that leaves the z missing. What does X & Y in the heighmap actually stand for? Is each value just a single value? For instance getHeighMap(0,3) represents only one number.... But where is that number referencing on the terrain?

Is the logic behind storing data in the heightmap akin to a standard x/y 2-dimensional grid such as you would find a basic chart?

Is this what the heighmap is:? ![alt text][1] [1]: http://whiteboardsetc.com/images/xy-axis.jpg

If so, I'm sorry for my ignorance, but since starting unity, everything Z is for depth and Y seams to be elevation. Maybe it's traditional in this sense? I may have actually answered my own question in this post, but please confirm my understanding if so.