from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. An element of a graphical user interface in the form of a box of text that appears when a cursor is made to hover over an item; normally used to explain the function of the item.
tool + tip, originally used with toolbars to provide a tip or hint as to what function each graphical icon represented. The term was introduced by Microsoft circa Windows 95. (Wiktionary)