WindowFloating

WindowFloating is an option for Notebook which specifies whether the window for the notebook should float on top of other windows when it is displayed on the screen.

WindowFloating->True is often used for palettes.

If there are several floating windows the most recently selected one goes on top.

See The Mathematica Book: Section 2.10.2 and Section 2.10.13.

See also: WindowClickSelect, Visible.