What does "DxD" in "Highschool DxD" stand for? I have been following High School DxD's anime and manga. However, there has been no explanation of what DxD in the title means so far.

Does anyone know what it stands for?
As revealed in Volume 16 of the light novel, DxD is the name of an anti-terrorism organization and has the meaning of the different factions which form the group:

Devils, also a Dragon, and the word “Fallen” of a Fallen Angel—like downfall.

(In Japanese, Fallen Angel is [堕天使]【だてんし】, whose romaji datenshi starts with D)
Quoting from the novel:

—Then Koneko-chan mutters.
“—[D×D].”
Everyone gazes at Koneko-chan’s muttering. Koneko-chan is surprised at
having so much attention directed towards her. She then continues
while feeling shy.
“Well, since it’s a mixed team consisted of different beings, that’s
how I felt……”
Rias then asks.
“What does the [D×D] stand for? Does it stand for being a Dragon of
Dragon like Great Red?”
“No, it can stand for Devils, also a Dragon, and the word “Fallen” of
a Fallen Angel—like downfall.”
Koneko-chan answers.
I see, so it means [D×D] while pointing towards Devils, Dragons, and
such.
Sensei nods his head.


According to Volume 6 of the light novel, DxD also stands for Dragon of Dragons, referring to the Great Red:

“There are two dragons called the ‘Red Dragons’. One of them is the ancient dragon from Wales residing in you. Welsh Dragon. The Sekiryuutei. Hakuryuukou also comes from the same origin and the same myth. But there is one more ‘Red Dragon’. That is the Red-Dragon which is recorded in Revelation.”
“Revelation……?”
“The True God-Emperor of Red-Dragon. Apocalypse Dragon, the Great-Red. It’s the great dragon which is called the ‘True-Dragon’. It chooses to live in the dimensional gap and swims there for eternity. Today, we came here to confirm ‘that’. The field of Rating Game is inside a section in the dimensional gap by putting on a barrier. This time, Ophis’s true aim was to confirm that. Shalba’s plan was something which wasn’t important to us.”
[...]
That time, Vali showed the clearest eyes ever seen before.
“The one I want to fight the most. The Apocalypse Dragon Great-Red who is called [DxD], Dragon of Dragon. –I want to become “The True God-Emperor of White-Dragon”. It won’t look good if the ‘white’ is a rank below the ‘Red’ when there is the ultimate-class for ‘Red’ right? That’s why I will become one. One day when I defeat Great-Red.”