passerby vs passer

passerby and passer both are nouns.

Word	Noun	Adjective	Verb	Adverb
passerby	Yes	No	No	No
passer	Yes	No	No	No

As nouns, passerby and passer are synonyms defined as:

Other synonyms of passerby include passer-by.

passerby (noun)	passer (noun)
a person who passes by casually or by chance	(football) a ball carrier who tries to gain ground by throwing a forward pass
	a person who passes by casually or by chance
	a student who passes an examination
	a person who passes as a member of a different ethnic or racial group