1.
outsider; third party; unauthorized person
A person who does not belong to a particular group, organization, or area, and therefore has no right to be involved or to enter.
Usage
部外者 literally means "person outside the department/section." It is commonly used on signs restricting access and in discussions about who has the right to be involved in a matter.
Common collocations
Similar words
第三者 (third party) is more neutral and legal, while 部外者 emphasizes not belonging to the group in question. よそ者 (stranger, outsider) is more colloquial.