Visible

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 The Mathematica Book: Section 2.10.13.

See also: Selectable, WindowFloating, CellOpen.