The parameters defined by the user that help determine how a printed page will appear. Those parameters can include everything from the size, margins, page orientation, to quality of print. More plainly, it's a menu that allows users to customize the layout of a page. The page setup dialog box is usually available from the File drop-down menu. For Microsoft Word 2007 or later, the page setup options are available under the Layout tab in the Ribbon menu. Below are some examples of how Page Setup may appear in your programs.
Page Setup in Microsoft Word
Alternative page setup window