Z-order
(重定向自Z order)
Z-order is an ordering of overlapping two-dimensional objects, such as windows in a graphical user interface (GUI), shapes in a vector graphics editor, or objects in a 3D application. One of the features of a typical GUI is that windows may overlap, so that one window hides part or all of another. When two windows overlap, their Z-order determines which one appears on top of the other.