Wikipedia
Xmouse is a system of mouse control used instead of the standard system of selection on computers (most notably Windows and X11, where it is an option). Where normal mouse control uses single-click for selection and double-click to open/edit/etc, the xmouse system automatically selects objects after hovering the mouse over the object for a certain period of time (often one second). The context function, previously activated by double-clicking is activated by single-clicking. Double-clicking is made redundant by this system and is ignored. Operation of menus, text selection and other features remain unchanged.
Proponents of the xmouse system claim it has many advantages over the regular system of selection. Most notably, it requires much less effort on the part of the user to use the computer, and can be considerably faster once users have adjusted to the non-standard interface.
The two main disadvantages are the selection speed, and the non-standardness of the xmouse system. Firstly, if the activated window is also configured to be drawn into foreground there must be a minimum time for the cursor to hover over a window before it is selected, or else all touched windows would be immediately in foreground and cover other windows. This 'lag time' reduces the maximum speed that xmouse can be used at, and could lead to annoying pauses. Secondly, as it is rarely used in the world, people who get used to the xmouse system might find it hard to adjust to using other computers, and vice versa.
On Microsoft Windows, an Xmouse Control Panel applet was available as part of the Windows 95 PowerToys, used to configure focusing windows in the user interface by hovering. On newer Windows versions, the behavior can be configured using Tweak UI or directly from the Windows Registry. The setting is also built into Windows Vista and Windows 7 in the Ease of Access Center as "Activate a window by hovering over it with the mouse", however preventing the activated window from moving to foreground can be done only by configuring the registry.