An oath (from Anglo-Saxon að, also called plight) is either a promise or a statement of fact calling upon something or someone that the oath maker considers sacred, usually a god, as a witness to the binding nature of the promise or the truth of the statement of fact. To swear is to take an oath.
A person taking an oath indicates this in a number of ways. The most usual is the explicit "I swear," but any statement or promise that includes "with * as my witness" or "so help me *," with '*' being something or someone the oath-taker holds sacred, is an oath. Many people take an oath by holding in their hand or placing over their head a book of scripture or a sacred object, thus indicating the sacred witness through their action: such an oath is called corporal. However, the chief purpose of such an act is for ceremony or solemnity, and the act does not of itself make an oath.
There is confusion between oaths and other statements or promises. The current Olympic Oath, for instance, is really a pledge and not properly an oath since there is only a "promise" and no appeal to a sacred witness. Oaths are also confused with vows, but really a vow is a special kind of oath.
In law, oaths are made by a witness to a court of law before giving testimony and usually by a newly-appointed government officer to the people of a state before taking office. In both of those cases, though, an affirmation can be usually substituted. A written statement, if the author swears the statement is the truth, the whole truth, and nothing but the truth, is called an affidavit. The oath given to support an affidavit is frequently administered by a notary public who will memorialize the giving of the oath by affixing her or his seal to the document. Breaking an oath (or affirmation) is perjury.
Bailey (1907) states:
We have, for instance, the sacred stone (silex) which was preserved in the temple of Iuppiter on the Capitol, and was brought out to play a prominent part in the ceremony of treaty-making. The fetial, who on that occasion represented the Roman people, at the solemn moment of the oath-taking, struck the sacrificial pig with the silex, saying as he did so, 'Do thou, Diespiter, strike the Roman people as I strike this pig here to-day, and strike them the more, as thou art greater and stronger.' Here no doubt the underlying notion is not merely symbolical, but in origin the stone is itself the god, an idea which later religion expressed in the cult-title specially used in this connection, Iuppiter Lapis.
Walter Burkert has shown that since Lycurgus of Athens (d. 324 BC), who held that "it is the oath which holds democracy together", religion, morality and political organization had been linked by the oath, and the oath and its prerequisite altar had become the basis of both
civil and criminal, as well as international law.
The first personage in the biblical tradition to take an oath is held to be Eliezer, the chief servant of Abraham, when the latter requested of the former that he not take a wife for his son Issac from the daughters of Canaan, but rather from among Abraham's own family. In the Judeo-Christian Tradition, this is held as the origination of the concept that it is required to hold a sacred object in one's hand when taking an oath. Because circumcision was the first commandment Abraham performed and was therefore so dear to him on both a spiritual as well as a personal level, Abraham had Eliezer take hold of his genitals.
Various religious groups have objected to the taking of oaths, most notably the Religious Society of Friends (Quakers) and Mennonites. This is principally based on , the Antithesis of the Law. Here, Christ is written to say "I say to you: 'Swear not at all'". The Apostle James stated in , "Above all, my brothers, do not swear—not by heaven or by earth or by anything else. Let your "Yes" be yes, and your "No," no, or you will be condemned."
Not all Christians follow this reading, because of the statements in the Old Testament. Jews also avoid taking oaths, as even making an unintentionally false oath would violate a Biblical commandment in .
Opposition to oath-taking caused many problems for these groups throughout their history. Quakers were frequently imprisoned because of their refusal to swear loyalty oaths. Testifying in court was also difficult; George Fox, Quakers' founder, famously challenged a judge who had asked him to swear, saying that he would do so once the judge could point to any Bible passage where Jesus or his apostles took oaths. (The judge could not, but this did not allow Fox to escape punishment.) Legal reforms from the 18th century onwards mean that everyone in the United Kingdom now has the right to make a solemn affirmation instead of an oath. The United States has permitted affirmations since it was founded; it is explicitly mentioned in the Constitution. Only two US Presidents, Franklin Pierce and Herbert Hoover, have chosen to affirm rather than swear at their inaugurations.
A prose passage inserted in the eddic poem Helgakviða Hjörvarðssonar relates:
Hedin was coming home alone from the forest one Yule-eve, and found a troll-woman; she rode on a wolf, and had snakes in place of a bridle. She asked Hedin for his company. "Nay," said he. She said, "Thou shalt pay for this at the bragarfull." That evening the great vows were taken; the sacred boar was brought in, the men laid their hands thereon, and took their vows at the bragarfull. Hedin vowed that he would have Sváva, Eylimi's daughter, the beloved of his brother Helgi; then such great grief seized him that he went forth on wild paths southward over the land, and found Helgi, his brother.Such Norse traditions are directly parallel to the "bird oaths" of late medieval France, such as the voeux du faisan (oath on the pheasant) or the (fictional) voeux du paon (oath on the peacock).