bystander (n.)

• abutter
• attestant
• attestator
• attester
• beholder
• borderer
• deponent
• eyewitness
• gazer
• informant
• informer
• kibitzer
• looker
• looker-on
• neighbor
• observer
• ogler
• onlooker
• passerby
• perceiver
• percipient
• seer
• spectator
• swearer
• tangent
• testifier
• viewer
• voucher
• watcher
• witness