A person outside a particular organization or group; someone not involved or authorized.
この会議は部外者の入室を禁止しています。
This meeting prohibits entry to outsiders.
部外者なので、詳しい事情は話せません。
Because I'm an outsider, I can't talk about the details.
現場は危険なので部外者は近づかないでください。
The site is dangerous, so outsiders should not approach.