According to Investopedia: A fiduciary is responsible for managing the assets of another person, or of a group of people. Asset managers, bankers, accountants, executors, board members, and corporate officers can all be considered fiduciaries when entrusted in good faith with the responsibility of managing another party's assets. 

According to Wikipedia: A fiduciary is a person who holds a legal or ethical relationship of trust with one or more other parties (person or group of persons). Typically, a fiduciary prudently takes care of money or other asset for another person.