An onlooker is someone who watches something happen without taking part in it. Picture a crowd gathering around two students arguing in the hallway: those students watching from the sides are onlookers. They're present, they can see what's happening, but they're not involved in the argument itself.

Onlookers might watch a street performer juggle, observe a science experiment from the back of the classroom, or witness a car accident from the sidewalk. The word emphasizes being on the outside looking in, seeing events unfold but staying separate from the action.

Sometimes people criticize onlookers who watch something wrong happen without helping or speaking up. For example, if someone is being teased and others just stand there watching, those people are onlookers instead of helpers or defenders. Other times, being an onlooker is perfectly fine: watching a parade, observing wildlife, or simply noticing something interesting as you pass by.

The word bystander means almost the same thing, though bystander often suggests someone who happens to be nearby, while onlooker emphasizes the act of watching.