User blog comment:B214/Talk About What You Want! 3/@comment-110.147.195.80-20150124231845/@comment-25935458-20150125021846

In volume 16, when the counter terrorist team is formed Dulio said it needed a name and Koneko said "DxD". when Rias asks "What does the [DxD} stand for? Does it stand for being a Dragon of Dragon like Great Red?" Koneko said "No, it can stand for Devils, also a Dragon, and the word "Fallen" of a Fallen Angel --like downfall". There is more said (feel free to reread it), but this is the essence of it.

That being said, and I could be wrong, but the author has not given another explanation as to what "DxD" really is. Untill Ishibumi states that "DxD" means "????????" it is open for your own interpretation.