The tooltip is a common [graphical user interface] element. It is used in conjunction with a [cursor], usually a mouse pointer. The user hovers the cursor over an item, without clicking it, and a small box appears with a name or description of the item being hovered over. A common variant, especially in older software, is displaying a description of the tool in a status bar, but such descriptions are not usually called tooltips.
A good example of tooltips are the widely used tooltips on web pages. Many graphical web browsers display the attribute "title" of an HTML element when it is hovered.
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