What is crawl delay in robots txt?

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Tags

1. #server
2. #adding
3. #crawl
4. #prevent
5. #engines
6. #requests
7. #differs
8. #overloading
9. #overload
10. #servers
11. #robots
12. #directive
13. #unofficial
14. #search
15. #delay
16. #temporary
17. #handle