onrush vs onset

onrush and onset both are nouns.

Word	Noun	Adjective	Verb	Adverb
onrush	Yes	No	No	No
onset	Yes	No	No	No

As nouns, onrush and onset are synonyms defined as:

onrush and onset: (military) an offensive against an enemy (using weapons)

Other synonyms of onrush include attack, onslaught.

onrush (noun)	onset (noun)
(military) an offensive against an enemy (using weapons)	(military) an offensive against an enemy (using weapons)
a forceful forward rush or flow	the beginning or early stages