collision vs pileup

collision and pileup both are nouns.

Word	Noun	Adjective	Verb	Adverb
collision	Yes	No	No	No
pileup	Yes	No	No	No

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

Other hyponyms of collision include fender-bender, smash-up, smash.

collision (noun)	pileup (noun)
a conflict of opposed ideas or attitudes or goals	multiple collisions of vehicles
an accident resulting from violent impact of a moving object
(physics) a brief event in which two or more bodies come together