For Defined Benefit (DB) plans, any person who exercises power and control, management or disposition with regard to a pension plan’s assets, or who has authority to do so or who has authority or responsibility for plan administration.
For Defined Contribution (DC) plans, any person with discretionary authority over plan administration or investments. Plan fiduciaries ordinarily include the employer, trustees, and investment advisers. Fiduciaries must discharge their duties solely in the interest of the plan participants and are accountable for any actions which may be construed by the courts as breaching that trust.