What does the "D" in D-Day stand for?
Nothing. Or maybe "Day."
It is widely believed that the "D" in D-Day stands for "designated," or "decision," or something else along those lines. These beliefs, while creative, are unfounded. According to the Department of Defense Dictionary of Military and Associated Terms, D-Day is simply "The unnamed day on which a particular operation commences or is to commence." Similarly, H-Hour is "The specific hour on D-day at which a particular operation commences."
You can find more information at the National D-Day Memorial Foundation site.