bystander
A person who is nearby and watching but not involved.
A bystander is someone who is present when something happens but isn't directly involved. If two students argue in the hallway while others watch without joining in, those watching are bystanders. They're there, they see what's happening, but they're standing by rather than participating.
A bystander at a street performance watches the juggler but isn't performing. A bystander in a conversation hears what's being said but isn't part of the discussion.
Bystanders face interesting choices. When something goes wrong, like someone being bullied or hurt, bystanders must decide whether to stay on the sidelines or step in to help. Sometimes bystanders call for help, speak up, or intervene. Other times they remain passive, which can feel safe in the moment but might not sit well with them later.
The word often appears in news reports: “Bystanders helped the injured person until paramedics arrived.” It's also used in discussions about responsibility: when everyone is just a bystander, problems can grow worse because no one acts.