Any type of attestation by which an individual signifies that he or she is bound in conscience to perform a particular act truthfully and faithfully; a solemn declaration of truth or obligation.
An individual's appeal to God to witness the truth of what he or she is saying or a pledge to do something enforced by the individual's responsibility to answer to God.
Similarly an affirmation is a solemn and formal declaration that a statement is true; however, an affirmation includes no reference to God so it can be made by someone who does not believe in God or by an individual who has conscientious objections against swearing to God. Provisions in state statutes or constitutions ordinarily allow affirmations to be made as alternatives to oaths.
In order for an oath to be legally effective, it must be administered by a public official. The law creating each public office and describing the duties of the official ordinarily indicates who is authorized to administer the oath of office. A spoken oath is generally sufficient; however, a written and signed oath can be required by law.
The most famous oath prescribed by law in the United States is the oath repeated by the president-elect upon taking the office of the presidency.