countermove vs onrush

countermove and onrush both are nouns.

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

As nouns, onrush is a hypernym of countermove; that is, onrush is a word with a broader meaning than countermove:

countermove: an attack by a defending force against an attacking enemy force in order to regain lost ground or cut off enemy advance units etc.
onrush: (military) an offensive against an enemy (using weapons)

Other hypernyms of countermove include attack, onset, onslaught.

countermove (noun)	onrush (noun)
an attack by a defending force against an attacking enemy force in order to regain lost ground or cut off enemy advance units etc.	(military) an offensive against an enemy (using weapons)
	a forceful forward rush or flow