walker vs passer-by

walker and passer-by both are nouns.

Word	Noun	Adjective	Verb	Adverb
walker	Yes	No	No	No
passer-by	Yes	No	No	No

As nouns, passer-by is a hyponym of walker; that is, passer-by is a word with a more specific, narrower meaning than walker:

walker (noun)	passer-by (noun)
an enclosing framework on casters or wheels; helps babies learn to walk	a person who passes by casually or by chance
a light enclosing framework (trade name Zimmer) with rubber castors or wheels and handles; helps invalids or the handicapped or the aged to walk
a shoe designed for comfortable walking
a person who travels by foot