An attack; an assault especially of an army.
"The sudden onset of enemy troops shattered our defenses within minutes."
In plain English: Onset is just when something starts to happen.
"The sudden onset of rain forced everyone to run for cover inside the nearest shop."
Usage: Use onset to describe the sudden beginning or arrival of something, such as a disease, storm, or trend. Avoid confusing it with effect when referring specifically to the start rather than the result.
To assault; to set upon.
"The pack's sudden onset left the lone dog trembling in fear."
The word "onset" comes from combining Old English on- and settan, originally meaning to impose or bear down upon someone. Over time, this sense of pressing in developed into the modern definition of an attack or beginning.