What does use of undeclared identifier mean in Xcode?

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Tags

1. #means
2. #project
3. #sliderlabel
4. #defined
5. #identifier
6. #might
7. #compiler
8. #variable
9. #property
10. #undefined
11. #visible
12. #learn
13. #check
14. #undeclared
15. #probably
16. #field
17. #answer
18. #implementation
19. #precise
20. #right