A gesture used in older versions of Windows CE, mostly on Handheld PCs that had a small Desktop. Similar to double-click, this gesture involves tapping a stylus twice on a touchscreen in quick succession. Requires more coordination than the analogous mouse gesture, and is generally not user friendly.
Microsoft has thankfully since abandoned the Desktop UI in Windows CE 3.0, leaving only a heavily-modified version of the Start Menu (which looks and works a lot like the Apple Menu) in its place. Casio has abandoned that further still, and instead uses an icon-based launcher similar to that used on Palm OS to select applications on most of its new devices. Double-tapping has been made all but obsolete; the only time I've seen it used is for ease of text selection. On Palm OS 3.5 (or earlier versions with a HackMaster hack) and Windows CE, you can double-tap a word to select it, and triple-tap to select the whole paragraph.