What is Z-order ?

[Ground]

I'm trying to understand how to read Qt documentation.
I'm reading the QWidget section and i don't understand the part when they say : Z-order ...?
Here is the extract :

Every widget is rectangular, and they are sorted in a Z-order. A widget is clipped by its parent and by the widgets in front of it.

Thks in advance, any help would be appreciated!

[trickkx]

Widgets can be placed on top of each other.
See Wikipedia for a definition of "z-order":http://en.wikipedia.org/wiki/Z-order

[Chris Kawa]

Basically it comes from a 3 dimensional axis naming convention. left/right position is the X coordinate, top/bottom is the Y coordinate and depth (closer/farther) is Z coordinate.

[Guigui]

As the others said, the Z coordinates represent the position of an object in terms of depth. When two windows are on top of each other, the window with a higher Z value will be placed on top of the other one, thus masking a part of it (or maybe entirely).

[Ground]

Great explanations, understand well now, thks all :) .(it reminds me now the z-index in html which is also about how the elements are arranged in terms of depth).