What does the 7 mean in a7 chord?

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Tags

1. #consisting
2. #seven
3. #major
4. #semitones
5. #seventh
6. #thirdan
7. #degree
8. #fifthan
9. #interval
10. #minor
11. #chord
12. #constructed
13. #often
14. #theory
15. #dominant
16. #perfect
17. #seventhan
18. #scale
19. #abbreviated