From on- + set.

• (RP) IPA: /ˈɒnˌsɛt/
• (America, Midland US, US South) IPA: /ˈɔnˌsɛt/
• (Northern US, or, cot-caught) IPA: /ˈɑnˌsɛt/

onset (plural onsets)

1. (archaic) An attack; an assault especially of an army.

   Synonyms: storming

   • c. 1596 (date written), William Shakespeare, “The Life and Death of King Iohn”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act II, scene i]:

     Heralds, from off our towers, we might behold
     From first to last, the onset and retire
     Of both your armies

   • 1800, William Wordsworth, Six thousand Veterans:

     Who on that day the word of onset gave.

   • 1950, C. S. Lewis, The Lion, the Witch and the Wardrobe:

     And Peter's tired army cheered, and the newcomers roared, and the enemy squealed and gibbered till the wood re-echoed with the din of that onset.

2. (medicine) The initial phase of a disease or condition, in which symptoms first become apparent.

   the onset of schizophrenia

3. (phonology) The initial portion of a syllable, preceding the syllable nucleus.

   Hypernyms: anlaut

   Holonym: syllable

   Coordinate terms: nucleus, coda, rhyme

4. (acoustics) The beginning of a musical note or other sound, in which the amplitude rises from zero to an initial peak.
5. A setting about; a beginning.

   Synonyms: start, beginning, Thesaurus:beginning

   the onset of puberty

   • 1625, Francis [Bacon], “Of Delays”, in The Essayes […], 3rd edition, London: […] Iohn Haviland for Hanna Barret, →OCLC ↗:

     There is surely no greater wisdom than well to time the beginnings and onsets of things.

6. (obsolete) Anything added, such as an ornament or as a useful appendage.
   • c. 1588–1593 (date written), William Shakespeare, “The Lamentable Tragedy of Titus Andronicus”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act I, scene i]:

     And will with deeds requite thy gentleness:
     And, for an onset, Titus, to advance
     Thy name and honourable family,
     Lavinia will I make my empress.

• French: assaut, attaque
• German: Angriff, Ansturm, Anschlag
• Portuguese: investida
• Russian: ата́ка
• Spanish: arremetida

• French: début, apparition
• German: Anzeichen
• Portuguese: anábase
• Spanish: aparición

• French: attaque
• German: Silbenansatz, Silbenanfang, Silbenanlaut
• Italian: attacco
• Portuguese: ataque
• Russian: при́ступ
• Spanish: ataque

• French: attaque
• German: Anstieg, Einschwingvorgang, Anstrich

• French: déclenchement, début

onset (onsets, present participle onsetting; simple past and past participle onset)

1. (obsolete) To assault; to set upon.
2. (obsolete) To set about; to begin.