A relatively recent addition to the world of computer
user interface, a tooltip is a
textual description of an
icon or other
widget, which appears when one holds the
mouse pointer over that widget. This permits the widget to be somewhat
self-documenting.
For instance, suppose you do not know what the icon of a rubber stamp means in the button bar of your graphical text editor. If the editor supports tooltips, then by holding the mouse pointer over that icon, you can summon a tooltip bearing the word "Paste". This tells you that the icon invokes the editor's paste function.
I first saw tooltips as such in a version of Microsoft Word, several years ago. They are prefigured somewhat by the Macintosh's Balloon Help feature, which does much the same thing (but is arguably both more flexible and more intrusive). They are now widely used in various X-based applications for Linux and Unix, as well as on Windows and Macintosh programs.