passerby vs walker

passerby and walker both are nouns.

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

As nouns, walker is a hypernym of passerby; that is, walker is a word with a broader meaning than passerby:

Other hypernyms of passerby include footer, pedestrian.

passerby (noun)	walker (noun)
a person who passes by casually or by chance	an enclosing framework on casters or wheels; helps babies learn to walk
	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