Z is for Z-Index
The Z-index is a parameter in CSS positioning - strictly speaking
its a property of an element. It allows you to stack elements the way
you stack layers in a graphics programme such as Photoshop, or The
Gimp, or Paint Shop Pro. The index is the bit that sets the order of
the elements when they are stacked one on top of the other. The lowest
number which can include negative numbers has the lowest priority,
that is, is at the bottom of the stack.
The name obviously derives from the third axis (z) in 3-dimensional
representational systems.