What does compression and rarefaction mean?

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Tags

1. #troughs
2. #particles
3. #crests
4. #where
5. #furthest
6. #rarefaction
7. #together
8. #region
9. #apart
10. #waves
11. #rarefactions
12. #compressions
13. #closest
14. #however
15. #compression
16. #longitudinal
17. #instead