surprise attack vs onrush

surprise attack and onrush both are nouns.

Word	Noun	Adjective	Verb	Adverb
surprise attack	Yes	No	No	No
onrush	Yes	No	No	No

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

Other hypernyms of surprise attack include attack, onset, onslaught.

surprise attack (noun)	onrush (noun)
an attack without warning	(military) an offensive against an enemy (using weapons)
	a forceful forward rush or flow