A rushing or setting upon; an attack; an assault; a storming; especially, the assault of an army.
- An onslaught; an assault.
- A beginning; a start: the onset of a cold.
- Linguistics The part of a syllable that precedes the nucleus. In the word nucleus (no͞oˈklē-əs), the onset of the first syllable is (n), the onset of the second syllable is (kl), and the last syllable has no onset.