A special key with the Microsoft Windows logo (on most PC keyboards) that activates the "Start" menu in the MS Windows shell GUI.
For those who shun the mouse, this is a great addition to have on a keyboard while accessing programs.
However, for gamers, the Windows key is the bane of all existence (Especially for MMORPG gamers, where one touch of the Windows key can crash the entire program.)
"Using the windows key, I can start Microsoft Word in .7 seconds."
"OMFG SORRY YOU ALL DIED I HIT MY GODDAMN WINDOWS KEY AND CRASHED AGAIN :("