A tooltip is a temporary window containing text which appears when the user positions the cursor over an element for a period. The appearance is slightly delayed and the text is usually short.
Tooltips are often used for brief help text and identification of GUI elements. For example the "X" button alongside the Filter area in the Process Browser tool in the LispWorks IDE has a tooltip "Clear filter". Tooltips can also be used to complete the display of partially hidden text, for example in the Debugger tool Backtrace view where the display of long variable values might be truncated.
You can implement tooltips for
CAPI User Guide (Macintosh version) - 30 Aug 2011