A fiduciary duty is a legal or ethical relationship of confidence or trust between two or more parties. Typically, a fiduciary prudently takes care of money for another person. One party, for example a corporate trust company or the trust department of a bank, acts in a fiduciary capacity to the other one, who for example has funds entrusted to it for investment.
A fiduciary duty is the highest standard of care at either equity or law. A fiduciary is expected to be extremely loyal to the person to whom he owes the duty the "principal": he must not put his personal interests before the duty, and must not profit from his position as a fiduciary, unless the principal consents.