Noun

1. A rushing or setting upon; an attack; an assault; a storming; especially, the assault of an army.
   • William Shakespeare,

     The onset and retire Of both your armies.

   • William Wordsworth,

     Who on that day the word of onset gave.

2. The initial phase of a disease or condition, in which symptoms first become apparent.
3. The initial portion of a syllable, preceding the syllable nucleus.
4. 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.
   • Francis Bacon,

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

6. Anything set on, or added, as an ornament or as a useful appendage.

Verb

1. To assault; to set upon.
2. To set about; to begin.