A bystander is a person who observes a conflict or unacceptable behavior. It might be something serious or minor, one-time or repeated, but the bystander knows that the behavior is destructive or likely to make a bad situation worse. But he or she is not taking part in it.

Innocent bystander means a viewer, watcher, onlooker, or a guiltless witness of a crime.

An active bystander is a person who is standing near and watching something that is happening and takes steps to give appropriate help.