Onset may refer to:

Onset (audio), the beginning of a musical note or sound
Onset, Massachusetts, village in the United States
Onset Island (Massachusetts), a small island located at the western end of the Cape Cod Canal
Interonset interval, a term in music
Syllable onset, a term in phonetics and phonology
The Onset, Liverpool indie rock group formed by Mike Badger of the Las
The Onset (album), a 2005 album by the band
USS Onset (SP-1224), a United States Navy patrol boat in commission from 1917 to 1918
"Onset", a 2019 song by Haiku Hands

See also
Offset (disambiguation)
A supernumerary  kidney is an additional kidney to the number usually present in an organism. This often develops as the result of splitting of the nephrogenic blastema, or from separate metanephric blastemas into which partially or completely reduplicated ureteral stalks enter to form separate capsulated kidneys; in some cases the separation of the reduplicated organ is incomplete (fused supernumerary kidney). Less than a hundred cases are known of one or two supernumerary kidneys. See also
Supernumerary body part

References
Luciano A. Favorito; Ana Raquel M. Morais (May–June 2012). "Evaluation of supernumerary kidney with fusion using magnetic resonance image". IBJU. 38 (3): 428–429. doi:10.1590/s1677-55382012000300018. PMID 22765860.