Onset may refer to:
- website parsing, the beginning of a musical note or sound
- FITML, a term in music
- iOS, a term in phonetics and phonology
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Onset, Massachusetts, village in the United States
- Onset Island (Massachusetts), a small island located at the western end of the Cape Cod Canal
- The Onset, Liverpool indie rock group formed by Mike Badger of the La's
- USS Onset (SP-1224), a United States Navy patrol boat in commission from 1917 to 1918
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