What does the "D" in D-day stand for?

The D simply stands for “day.” The designation was traditionally used for the date of any important military operation or invasion, according to the National World War II Museum. Thus, the day before June 6, 1944, was known as D-1 and the days after were D+1, D+2, D+ and so on.

I HEARD IT STOOD FOR DOOMS DAY A LONG TIME AGO. 

"dominatrix". wait, that is my girlfriend. nevermind

D stands for Day.  day-day.  D-day stands for the day that H-hour happens on.  We are more likely to use the term Zero hour instead of H-hour

To me it almost says "that is day with a capital D"  where Day with a capital would mean it is a proper noun.  Like D-day is a specific day, not just any old day.

I thought it meant defeat (of the enemy), but I'm not sure.

D-Day stands for the First Day of the Battle of Allied Forces in Normandy ,France.It means a very important and secret military operation planned to be launched on that selected date which the headquarters think is the most appropriate after many,many considerations and fights in those seemingly endless meetings.

DAY. D Day was the first day of the allied invasion of Europe.

The landings that took place on the beaches of Normandy were D-1(Day one) The first day of the invasion. Each day that followed that was D2, D3, etc.