A person who is passing by (that is, walking past).(noun)
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Use "passer-by" in a sentence
"Tomlinson was not a demonstrator -- he was a passer-by on the way home."
"This tiny storefront could be easily overlooked by a passer-by given the dozens of Middle Eastern restaurants and stores on that block."
"Once, at a sharp turn where a man's shoulder would unavoidably brush against a screen of leaves, the bushman displayed great caution as he spread the leaves aside and exposed the head of a sharp-pointed spear, so set that the casual passer-by would receive at the least a nasty scratch."