An oath of fealty, from the Latin fidelitas (faithfulness), is a pledge of allegiance of one person to another. Typically the oath is made upon a religious object such as a Bible or saint's relic, often contained within an altar, thus binding the oath-taker before God.
In medieval Europe, fealty was sworn between two people, the obliged person (vassal) and a person of rank (lord). This was done as part of a formal commendation ceremony to create a feudal relationship.
Fealty and homage are a key element of feudalism.
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.