onset vs diversion

onset and diversion both are nouns.

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

As nouns, diversion is a hyponym of onset; that is, diversion is a word with a more specific, narrower meaning than onset:

onset: (military) an offensive against an enemy (using weapons)
diversion: an attack calculated to draw enemy defense away from the point of the principal attack

onset (noun)	diversion (noun)
(military) an offensive against an enemy (using weapons)	an activity that diverts or amuses or stimulates
the beginning or early stages	a turning aside (of your course or attention or concern)
	an attack calculated to draw enemy defense away from the point of the principal attack