A person or company that has contractually agreed, or a court has appointed, to manage the money or property of another. For example, a trustee or an executor of an estate.
Merriam-Webster Online Dictionary
fiduciary (noun) one that holds a fiduciary relation or acts in a fiduciary capacity
of, relating to, or involving a confidence or trust as
a) held or founded in trust or confidence
b) holding in trust
c) depending on public confidence for value or currency - fiduciary fiat money