If you witness a car accident while you're walking to school, you're a passer-by, and if you stroll past someone who's juggling flaming torches on the sidewalk, you're also a passer-by. Before the mid-sixteenth century, the now-obsolete passager had the same meaning. Today, passer-by is a fairly straightforward word ("one who passes by"); its only slightly confusing aspect being the plural form, which is passers-by.