What does unique viewers mean on YouTube?

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Tags

1. #viewers
2. #unique
3. #watch
4. #counts
5. #period
6. #viewer
7. #mobile
8. #whether
9. #picture
10. #number
11. #person
12. #desktop
13. #videos
14. #clearer
15. #audience
16. #different
17. #estimated
18. #given
19. #people