Wiktionary

1. z-order(Noun)

   The order in which objects should be drawn to display correctly.

   When you click on a window it is brought to the top of the z-order so that you can see it.

2. z-order(Noun)

   A property of a graphical object that indicates its position in the z-order.

   To get the rectangle to hide some of the text, make its z-order higher.

Origin: Analogous to z-axis.