These are all words for a person who sees sth happen. 以上各词均指目睹事情发生的人。

• witness a person who sees sth happen and is able to describe it to other people; a person who gives evidence in a court of law 指目击者、见证人、证人：◆Police have appealed for witnesses to the accident. 警方呼吁这起事故的目击者出来作证。
• observer a person who sees sth happen 指观察者、目击者：◆According to observers, the plane exploded shortly after take-off. 据目击者说，飞机起飞后不久就爆炸了。
• onlooker a person who watches sth that is happening but is not involved in it 指旁观者：◆A crowd of onlookers gathered at the scene of the crash. 在撞车地点聚集了一大群围观者。
• passer-by a person who is going past sb/sth by chance, especially when sth unexpected happens 指路人、过路的人，尤指意想不到的事发生时碰巧路过的：◆Police asked passers-by if they had witnessed the accident. 警察询问过路的人是否目击了这次事故。
• bystander a person who is near and can see what is happening when sth such as an accident or fight takes place 指现场目击者、旁观者：◆Three innocent bystanders were killed in the crossfire. 三名无辜的旁观者在交火中丧生。
• eyewitness a person who has seen a crime or accident and can describe it afterwards 指犯罪或事故现场的目击者、见证人

Patterns

• a witness/an observer/an onlooker/a passer-by/a bystander/an eyewitness sees sth
• an observer/an onlooker/a passer-by/a bystander witnesses sth