Legal Definition of Fiduciary: A person or organization who is given authority and responsibility over assets not owned by that fiduciary, such as an executor or personal representative in an estate, a trustee for a trust, or a conservator in a conservatorship | Looking for a clear understanding of the legal definition of fiduciary? Learn the roles and responsibilities of a fiduciary in managing assets and making decisions in the best interests of beneficiaries. Discover the importance of fiduciary responsibility and how to choose a trustworthy and knowledgeable individual or organization