Visible is an option for Notebook which specifies whether the notebook should be explicitly displayed on the screen.
With Visible->False a notebook can still be manipulated from the kernel, but will not explicitly be displayed on the screen.
NotebookCreate[Visible->False] creates a new invisible window.
See Section 2.11.13.
See also: Selectable, WindowFloating, CellOpen.
New in Version 3.