Z-order Definition

noun
(computer graphics) 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.
Wiktionary
(computer graphics) 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.
Wiktionary