1. passerby (复数为passersby)。
passerby 意为行人，源于pass by 。例句：
Domestic media also tracked down some of the passersby in the video.
2. bystander。
意为旁观者，源于stand by。例句：
"This is sort of classic bystander effect, where people have a diffusion of responsibility," Wendy Walsh, a psychologist based in Los Angeles, commented on CNN.
以上例句源于CNN：http://edition.cnn.com/2011/10/18/world/asia/china-toddler-hit-and-run/