The fiduciary duty is a legal relationship between two or more parties (most commonly a "fiduciary" or "trustee" and a "principal" or "beneficiary") that in English common law is arguably the most important concept within the portion of the legal system known as equity. In the United Kingdom, the Judicature Acts merged the courts of Equity (historically based in England's Court of Chancery) with the courts of common law, and as a result the concept of fiduciary duty also became usable in common law courts.
A fiduciary duty is the highest standard of care imposed at either equity or law. A fiduciary is expected to be extremely loyal to the person to whom they owe the duty (the "principal"): they must not put their personal interests before the duty, and must not profit from their position as a fiduciary, unless the principal consents. The fiduciary relationship is highlighted by good faith, loyalty and trust, and the word itself originally comes from the Latin fides, meaning faith, and fiducia.
When a fiduciary duty is imposed, equity requires a stricter standard of behavior than the comparable tortious duty of care at common law. It is said the fiduciary has a duty not to be in a situation where personal interests and fiduciary duty conflict, a duty not to be in a situation where their fiduciary duty conflicts with another fiduciary duty, and a duty not to profit from their fiduciary position without express knowledge and consent. A fiduciary cannot have a conflict of interest. It has been said that fiduciaries must conduct themselves "at a level higher than that trodden by the crowd."