adj. Of or relating to a holding of something in trust for another: a fiduciary heir; a fiduciary contract. adj. Of or being a trustee or trusteeship.adj. Held in trust.adj. Of or consisting of fiat money.adj. Of, relating to, or being a system of marking in the field of view of an optical instrument that is used as a reference point or measuring scale.n. One, such as an agent of a principal or a company director, that stands in a special relation of trust, confidence, or responsibility in certain obligations to others.