rar·e·fac·tion 
(râr′ə-făkshən)
(râr′ə-făkshən)n.
1. The condition of being rarefied.
2.
a. A decrease in density and pressure in a medium, such as air, caused by the passage of a sound wave.
b. The region in which this occurs.
rar′e·factive adj.
- setup pitcher
- set up shop
- Setúbal
- Seurat
- Seuss
- Sevastopol
- seven
- Seven Hills of Rome
- seven seas
- seventeen
- seventeenth
- seventeen-year locust
- seventh
- Seventh Avenue
- Seventh-day Adventist
- seventh heaven
- seventh-inning stretch
- seventieth
- seventy
- seventy-eight
- seven-up
- Seven Wonders of the World
- sever
- severability
- severable