What does direct message mean on Facebook?

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Tags

1. #message
2. #recipient
3. #communication
4. #sender
5. #media
6. #between
7. #direct
8. #messages
9. #twitter
10. #varying
11. #users
12. #restrictions
13. #definition
14. #platform
15. #facebook
16. #visible
17. #private
18. #allow
19. #social
20. #instagram
21. #their