real party in interest

Definition from Nolo’s Plain-English Law Dictionary

The person or entity who will benefit from a lawsuit or petition even though the plaintiff (the person filing the suit) is someone else (often called a "nominal" plaintiff). For example, a trustee files a suit against a person who damaged a building owned by the trust; the real party in the interest is the beneficiary of the trust.

Definition provided by Nolo’s Plain-English Law Dictionary.