A passerby is someone who happens to be walking or traveling past a particular place at a particular moment. If you're standing on a street corner and people walk by without stopping, they're passersby.

Passersby play important roles in many situations. A street musician performs for passersby, hoping they'll stop to listen or drop coins in a hat. When someone needs help, a helpful passerby might call 911 or offer assistance. Witnesses to crimes or accidents are often passersby who just happened to be in the area.

The word emphasizes the temporary, coincidental nature of being somewhere. A passerby isn't visiting that location on purpose or staying there. They're simply moving through. If you drop your backpack and a passerby helps you pick up your scattered papers, that person wasn't planning to help you that morning. They were just walking past at the right moment.

The plural is passersby, not “passersbys.” You might read in a news story that “several passersby witnessed the event” or that “passersby stopped to admire a window display.”