What does DxD mean in high school DXD?

Related questions

1. When did high school DxD season 4 come out?
2. Will there be a fourth season of highschool DxD?
3. Is there a season 5 of highschool DxD?
4. Will there be a new high school DxD?

Tags

1. #angel
2. #terrorism
3. #which
4. #revealed
5. #fallen
6. #novel
7. #meaning
8. #organization
9. #light
10. #factions
11. #devils
12. #dragon
13. #different
14. #volume
15. #downfall
16. #group