A behavior of some windowing systems and particularly of certain window manager configurations under the X Window System. Causes the window manager to deliver keystrokes to whatever window the mouse pointer happens to be over. Also known as "focus follows mouse."

In other words, if your window manager is set to behave in this way, then you must keep your mouse pointer over the xterm or other window into which you are typing. Moving it to another window will cause your typing to register there instead. In order to raise a window to the foreground, you must either click on it, or in some window managers, leave your mouse pointer over it for a few seconds.

Some people prefer focus-follows-mouse behavior because it lets them send keystrokes to several different windows quickly, without raising or having to click on any of them. I, for one, am more used to focus follows click (aka click-to-focus) behavior, and am prone to momentary disorientation when I have to work on a focus-follows-mouse system.

Also, the general behavior of the optical apparatus of a domestic feline in the presence of Mus musculus.

Update, July 2001: I now use focus-follows-mouse behavior almost exclusively when using GNU/Linux and other Unix systems, and sometimes miss it on my Macintosh. Why the change? I'm not sure, but it came at the same time I switched from GNOME and Netscape Navigator to KDE and Konqueror.