The pushpin control to dock or undock window in ArcMap or ArcCatalog is not visible, so the window cannot be hidden or displayed to the side as a tab.
Note: The content in this article pertains to ArcGIS versions 10.0. Later versions of ArcGIS may contain different functionality, as well as different names and locations for menus, commands and geoprocessing tools.
The window has been dragged away from the ArcMap or ArcCatalog user interface and is not docked, so the window cannot be hidden.
Dock the window. Double-click the title bar of the window to dock it. The pushpin control is now visible.
See the section 'New dockable window controls make it easy to arrange and organize your display' in the 'What's new in ArcMap 10 basics' Help topic and the 'ArcMap: Usability Improvements' video in the Related Information section below.