1. In general, the entity that generally but not necessarily benefits from the judgment of a law suit, and is legally entitled to enforce the right that is subject matter of the suit. 2. In employee benefit plans, the individual recognized under the Employee Retirement Income Security Act (ERISA) as having a relationship with a retirement plan that prohibits him or her from engaging in certain transactions with the plan. Such persons usually include counsels, custodians, fiduciaries, plan sponsors, service providers, and trustees. also called real party in interest.