Refers to a person not belonging to a particular group, circle, or situation; often used in contexts like organizations, discussions, or events.
He participated in the meeting as an outsider.
第三者 means 'third party' and is more neutral/legal, often used in contracts or disputes, whereas 局外者 implies being outside a particular circle or situation.
Compound of 局外 (outside the bureau/circle) + 者 (person). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning 'outsider'.