An oath of fealty, from the Latin fidelitas (faithfulness), is a pledge of allegiance of one person to another.
In medieval Europe, the swearing of fealty took the form of an oath made by a vassal, or subordinate, to his lord. "Fealty" also referred to the duties incumbent upon a vassal that were owed to the lord, which consisted of service and aid. One part of the oath of fealty included swearing to always remain faithful to the lord. The oath of fealty usually took place after the act of homage, when, by the symbolic act of kneeling before the lord and placing his hands between the hands of the lord, the vassal became the "man" of the lord. Usually, the lord also promised to provide for the vassal in some form, either through the granting of a fief or by some other manner of support. Typically the oath took place upon a religious object such as a Bible or saint's relic, often contained within an altar, thus binding the oath-taker before God. Fealty and homage were key elements of European feudalism.
Fealty is distinct from other parts of the homage ceremony, and is usually used only to refer to that part of the ceremony where the vassal swore to be a good vassal to his lord.
In medieval Europe, an oath of fealty (German: Lehnseid) was a fundamental element of the feudal system in the Holy Roman Empire. It was sworn between two people, the obliged person (vassal) and a person of rank (liege lord). The oath of allegiance was usually carried out as part of a traditional ceremony in which the liegeman or vassal gave his lord a pledge of loyalty and acceptance of the consequences of a breach of trust. In return the liege lord promised to protect and remain loyal to his vassal. The rights conferred on the vassal were so similar to actual possession that it was described as beneficial ownership (dominium utile), whereas the rights of the lord were referred to as direct ownership (dominium directum).
In the Late Middle Ages, the investiture and oath of fealty were invariably recorded by a deed; in modern times this replaced the traditional ceremony. Where the geographical distance between the two parties was significant, the lord could name a representative before whom the oath was to be sworn.
The whole contract including the oath of fealty was part of a formal commendation ceremony that created the feudal relationship.
The term is also used by English-speakers to refer to similar oaths of allegiance in other feudal cultures, as with medieval Japan, as well as in modern political contexts.