(ὅρκος). An oath, either formally and solemnly pronounced as a part of a religious or legal act, or loosely used in common life. Among the Greeks oaths were taken on important occasions, such as treaties, alliances, and other compacts, in making vows, in ratifying promises; and on many other occasions, such as the registry of a youth as an Athenian citizen (see Ephebi), the assumption of public office by kings and ephors in Sparta, and by archons, generals, judges, and other officials at Athens, by judges of the games, by both accuser and defendant in the ἀνάκρισις (see Anacrisis), and by witnesses (see Martyria).
Oaths were sworn in the name of all the gods together, or by the special deity appropriate to the special occasion, as when virgins swore by Artemis, married women by Her, etc. Women had also their own peculiar oaths, μὰ τὼ θεώ (by the two goddesses, i. e. Demeter and Persephon), νὴ τὴν Ἀφροδίτην, etc. The Spartans swore by Castor and Pollux as their national demigods, and the Athenians by their deities (Zeus, Poseidon, and Athen), much as in the age of chivalry the nations of Europe swore by their national saints. The Greeks also swore by whatever was especially dear to theme. g. Achilles by his sceptre, warriors by their weapons, friends by their friendship. The most solemn oaths, however, were those that were sworn by Zeus (hence styled ὅρκιος), who punished perjury both in this life and after death ( Il.xix. 260; Ol. ii. 71). The gods themselves swore by the Styx as the most terrible of oaths ( Il.xv. 36 foll.). In taking an oath men usually stood, lifting up the hands and eyes to heaven, the oath being a sort of prayer; and sacrifices and libations were the accompaniments of the most solemn adjurations. To give them especial sanctity, oaths were often taken in temples, shrines, or sacred groves.
Among the Romans, oaths were taken by the magistrates and other persons who entered the service of the State, by soldiers, by the fetiales in ratifying treaties with foreign powers, by the parties to a legal action before the praetor, by witnesses, and loosely by persons in the ordinary speech of common life. Under the Empire, oaths of allegiance were commonly taken by all subjects of the emperor, and on the recurrence of the day on which such an oath had been taken, it was customary to renew it, and also on the first day of each year. The form of this oath of allegiance was probably the same as that of the sacramentum (see below). Vestal virgins and the Flamen Dialis were not allowed to swear on any occasion. Men swore by the gods, much as did the Greeks, by their genii, or by the genius of the emperor, and also by individuals or objects most dear to them. Favorite oaths were Hercle or Mehercle (i. e. ita me Hercules iuvet); Pol, Perpol, Aedepol (i. e. per Pollucem, ah di Pollux); per Iovem and per Iovem Lapidem; per superos; per deos immortales; medius fidius (i. e. ita me deus Fidius iuvet); ita me dii ament. Women never swore by Hercules nor men by Castor ( Gell.xi. 6). The form called exsecratio was common. Thus, di me perdant ni . . .; dispeream; di me interficiant; ne vivam; ne salvus sim, etc.
The oath taken in legal actions was called iusiurandum in iure and also iusiurandum necessarium as being required, and hence opposed to the iusiurandum voluntarium out of court. The oath taken by the Roman soldier was called sacramentum. It was taken on enlisting for a campaign, and first gave the soldier the right of using arms against the enemy. It thus distinguished legitimate warfare (militia) from mere rapine (latrocinium). It contained an exsecratio, and he who broke it was accursed (sacer). The formula of a sacramentum is given by Gellius (xvi. 4).
The ancients, especially the Greeks, tried to check the indiscriminate use of oaths in daily life. To allow men to relieve their feelings without profanity, Rhadamanthus is said to have ordained swearing by the names of animals, much as in modern times resort is had to such expressions as By Jingo! Thunder! Great Scott! etc. Thus Socrates swore by the dog, Lampon by the goose, and such elliptical expressions as μὰ τὸν (with the name of the deity suppressed) were common, just as in English men say I swear! ( Ran. 1374). See Lasaulx, Ueber den Eid bei den Griechen; Brissonius, De Formulis, etc.; and the article Sponsio.