affected party; party concerned
Formal term used in legal, business, or official contexts to refer to a person or entity directly involved in a matter.
Regarding this issue, the consent of all affected parties is required.
Entry by anyone other than the parties concerned is prohibited.
当事者 is a more general term for 'party' or 'person concerned', while 関係当事者 explicitly emphasizes the relationship or involvement in a specific matter.
Compound of 関係 (relation) and 当事者 (party concerned).