1. the beginning or start of something, especially something unpleasant
2. a sudden attack or assault
Usage: military; formal

Examples

• The onset of winter brought heavy snowfall to the region.
• She felt dizzy at the onset of her illness.
• The onset of the economic crisis caught many businesses unprepared.
• Doctors can now detect the onset of Alzheimer’s disease earlier.
• The enemy’s sudden onset overwhelmed the defenders.
• At the onset of the meeting, the chairman outlined the agenda.
• The onset of puberty varies from child to child.