What is Oath?

Legal Definition
Traditionally an oath (from Anglo-Saxon āð, also called plight) is either a statement of fact or a promise with wording relating to something considered sacred as a sign of verity. A common legal substitute for those who conscientiously object to making sacred oaths is to give an affirmation instead. Nowadays, even when there's no notion of sanctity involved, certain promises said out loud in ceremonial or juridical purpose are referred to as oaths. To swear is a verb used to describe the taking of an oath, to making a solemn vow.
-- Wikipedia
Legal Definition
A declaration made according to law, before a competent tribunal or officer, to tell the truth; or it is the act of one who, when lawfully required to tell the truth, takes God to witness that what he says is true. It is a religious act by which the party invokes God not only to witness the truth and sincerity of his promise, but also to avenge his imposture or violated faith, or in other words to punish his perjury if he shall be guilty of it. l0 Toull. n. 343 a 348; Puff. book, 4, c. 2, s. 4; Grot. book 2, c. 13, s. 1; Ruth Inst. book 1, ch. 14, s. 1; 1 Stark. Ev. 80; Merl. Repert. Convention; Dalloz, Dict. Serment: Dur. n. 592, 593; 3 Bouv. Inst. n. 3180.

2. It is proper to distinguish two things in oaths; 1. The invocation by which the God of truth, who knows all things, is taken to witness. 2. The imprecation by which he is asked as a just and all-powerful being, to punish perjury.

3. The commencement of an oath is made by the party taking hold of the book, after being required by the officer to do so, and ends generally with the words,"so help you God," and kissing the book, when the form used is that of swearing on the Evangelists. 9 Car. & P. 137.

4. Oaths are taken in various forms; the most usual is upon the Gospel by taking the book in the hand; the words commonly used are, "You do swear that, " &c. "so help you God," and then kissing the book. The origin of this oath may be traced to the Roman law, Nov. 8, tit. 3; Nov. 74, cap. 5; Nov. 124, cap. 1; and the kissing the book is said to be an imitation of the priest's kissing the ritual as a sign of reverence, before he reads it to the people. Rees, Cycl. h. v.

5. Another form is by the witness or party promising holding up his right hand while the officer repeats to him,"You do swear by Almighty God, the searcher of hearts, that," &c., "And this as you shall answer to God at the great day."

6. In another form of attestation commonly called an affirmation, (q. v.) the officer repeats, "You do solemnly, sincerely, and truly declare and affirm, that," &c.

7. The oath, however, may be varied in any other form, in order to conform to the religious opinions of the person who takes it. 16 Pick. 154, 156, 157; 6 Mass. 262; 2 Gallis. 346; Ry. & Mo. N. P. Cas. 77; 2 Hawks, 458.

8. Oaths may conveniently be divided into promissory, assertory, judicial and extra judicial.

9. Among promissory oaths may be classed all those taken by public officers on entering into office, to support the constitution of the United States, and to perform the duties of the office.

10. Custom-house oaths and others required by law, not in judicial proceedings, nor from officers entering into office, may be classed among the assertory oaths, when the party merely asserts the fact to be true.

11. Judicial oaths, or those administered in judicial proceedings.

12. Extra-judicial oaths are those taken without authority of law, which, though binding in foro conscientiae, do not render the persons who take them liable to the punishment of perjury, when false.

13. Oaths are also divided into various kinds with reference to the purpose for which they are applied; as oath of allegiance, oath of calumny, oath ad litem, decisory oath, oath of supremacy, and the like. As to the persons authorized to administer oaths, see Gilp. R. 439; 1 Tyler, 347; 1 South. 297; 4 Wash. C. C. R. 555; 2 Blackf. 35.

14. The act of congress of June 1, 1789, 1 Story's L. U. S. p. 1, regulates the time and manner of administering certain oaths as follows:

§1. Be it enacted, &c., That the oath or affirmation required by the sixth article of the constitution of the United States, shall be administered in the form following, to wit, "I, A B, do solemnly swear or affirm, (as the case may be,) that I will support the constitution of the United States." The said oath or affirmation shall be administered within three days after the passing of this act, by any one member of the senate, to the president of the senate, and by him to all the members, and to the secretary; and by the speaker of the house of representatives, to all the members who have not taken a similar oath, by virtue of a particular resolution of the said house, and to the clerk: and in case of the absence of any member from the service of either house, at the time prescribed for taking the said oath or affirmation, the same shall be administered to such member when he shall appear to take his seat.

15. - §2. That at the first session of congress after every general election of representatives, the oath or affirmation aforesaid shall be administered by any one member of the house of representatives to the speaker; and by him to all the members present, and to the clerk, previous to entering on any other business; and to the members who shall afterwards appear, previous to taking their seats. The president of the senate for the time being, shall also administer the said oath or affirmation to each senator who shall hereafter be elected, previous to his taking his seat; and in any future case of a president of the senate, who shall not have taken the said oath or affirmation, the same shall be administered to him by any one of the members of the senate.

16. - §3. That the members of the several state legislatures, at the next session of the said legislatures respectively, and all executive and judicial officers of the several states, who have been heretofore chosen or appointed, or, who shall be chosen or appointed before the first day of August next, and who shall then be in office, shall, within one month thereafter, take the same oath or affirmation, except where they shall have taken it before which may be administered by any person authorized by the law of the state, in which such office shall be holden, to administer oaths. And the members of the several state legislatures, and all executive and judicial officers of the several states, who shall be chosen or appointed after the said first day of August, shall, before they proceed to execute the duties of their respective offices, take the foregoing oath or affirmation, which shall be administered by the person or persons, who, by the law of the state, shall be authorized to ad- minister the oath of office; and the person or persons so administering the oath hereby required to be taken, shall cause a record or certificate thereof to be made, in the same manner as, by the law of the state, he or they shall be directed to record or certify the oath of office.

17. - §4. That all officers appointed or hereafter to be appointed, under the authority of the United States, shall, before they act in their respective offices, take the same oath or affirmation, which shall be administered by the person or persons who shall be authorized by law to administer to such officers their respective oaths of office; and such officers shall incur the same penalties in case of failure, as shall be imposed by law in case of failure in taking their respective oaths of office.

18. - §5. That the secretary of the senate, and the clerk of the house of repreentatives, for the time being, shall, at the time of taking the oath or affirmation aforesaid, each take an oath or affirmation in the words following, to wit; "I, A B, secretary of the senate, or clerk of the house of representatives (as the case may be) of the United States of America, do solemnly swear or affirm, that I will truly and faithfully discharge the duties of my said office to the best of my knowledge and abilities."

19. There are several kinds of oaths, some of which are enumerated by law.

>20. Oath of calumny. This term is used in the civil law. It is an oath which a plaintiff was obliged to take that he was not actuated by a spirit of chicanery in commencing his action, but that he had bona fide a good cause of action. Poth. Pand. lib. 5, t. 16 and 17, s. 124. This oath is somewhat similar to our affidavit of a cause of action. Vide Dunlap's Adm. Pr. 289, 290.

21. No instance is known in which the oath of calumny has been adopted in practice in the admiralty courts of the United States; Dunl. Adm. Pr. 290; and by the 102d of the rules of the district court for the southern district of New York, the oath of calumny shall not be required of any party in any stage of a cause. Vide Inst. 4, 16, 1; Code, 2, 59, 2; Dig. 10, 2, 44; 1 Ware's R. 427.

22. Decisory oath. By this term in the civil law is understood an oath which one of the parties defers or refers back to the other, for the decision of the cause.

23. It may be deferred in any kind of civil contest whatever, in questions of possession or of claim; in personal actions and in real. The plaintiff may defer the oath to the defendant, whenever he conceives he has not sufficient proof of the fact which is the foundation of his claim; and in like manner, the defendant may defer it to the plaintiff when he has not sufficient proof of his defence. The person to whom the oath is deferred, ought either to take it or refer it back, and if he will not do either, the cause should be decided against him. Poth. on Oblig. P. 4, c. 3, s. 4.

24. The decisory oath has been practically adopted in the district court of the United States, for the district of Massachusetts, and admiralty causes have been determined in that court by the oath decisory; but the cases in which this oath has been adopted, have been where the tender has been accepted; and no case is known to have occurred there in which the oath has been refused and tendered back to the adversary. Dunl. Adm. Pr. 290, 291.

25. A judicial oath is a solemn declaration made in some form warranted by law, before a court of justice or some officer authorized to administer it, by which the person who takes it promises to tell the truth, the whole truth, and nothing but the truth, in relation to his knowledge of the matter then under examination, and appeals to God for his sincerity.

26. In the civil law, a judicial oath is that which is given in judgment by one party to another. Dig. 12, 2, 25.

27. Oath in litem, in the civil law, is an oath which was deferred to the complainant as to the value of the thing in dispute on failure of other proof, particularly when there was a fraud on the part of the defendant, and be suppressed proof in his possession. See Greenl. Ev. §348; Tait on Ev. 280; 1 Vern. 207; 1 Eq. Cas. Ab. 229; 1 Greenl. R. 27; 1 Yeates, R. 34; 12 Vin. Ab. 24. In general the oath of the party cannot, by the common law, be received to establish his claim, but to this there are exceptions. The oath in litem is admitted in two classes of cases: 1. Where it has been already proved, that the party against whom it is offered has been guilty of some fraud or other tortious or unwarrantable act of intermeddling with the complainant's goods, and no other evidence can be had of the amount of damages. As, for example, where a trunk of goods was delivered to a shipmaster at one port to be carried to another, and, on the passage, he broke the trunk open and rifled it of its contents; in an action by the owners of the goods against the shipmaster, the facts above mentioned having been proved aliunde, the plaintiff was held, a competent witness to testify as to the contents of the trunk. 1 Greenl. 27; and see 10 Watts, 335; 1 Greenl. Ev. §348; 1 Yeates, 34; 2 Watts, 220; 1 Gilb. Ev. by Lofft, 244. 2. The oath in litem is also admitted on the ground of public policy, where it is deemed essential to the purposes of justice. Tait on Ev. 280. But this oath is admitted only on the ground of necessity. An example may be mentioned of a case where a statute can receive no execution, unless the party interested be admitted as a witness. 16 Pet. 203.

28. A promissory oath is an oath taken, by authority of law, by which the party declares that he will fulfil certain duties therein mentioned, as the oath which an alien takes on becoming naturalized, that he will support the constitution of the United States: the oath which a judge takes that he will perform the duties of his office. The breach of this does not involve the party in the legal crime or punishment of perjury.

29. A suppletory oath in the civil and ecclesiastical law, is an oath required by the judge from either party in a cause, upon half proof already made, which being joined to half proof, supplies the evidence required to enable the judge to pass upon the subject. Vide Str. 80; 3 Bl. Com. 270.

30. A purgatory oath is one by which one destroys the presumptions which were against him, for he is then said to purge himself, when he removes the suspicions which were against him; as, when a man is in contempt for not attending court as a witness, he may purge himself of the contempt, by swearing to a fact which is an ample excuse. See Purgation.
-- Bouviers Law Dictionary
Legal Definition
An external pledge or asseveration, made in verification of statements made or to be made, coupled with an appeal to a sacred or venerated object, In evidence of the serious and reverent state of mind of the party or with an invocation to a supreme being to witness the words of the party and to visit him with punishment lf they be false. See O'Reilly v. People, 86 N. Y. 154, 40 Am. Rep. 525; Atwood v. Welton, 7 Conn. 70; Clinton v. State, 33 Ohio St. 32; Brock v. Milligan. 10 Ohio, 123; thicker v. Burness, 2 Ala. 354. A religious asseveration, by which a person renounces the mercy and imprecates the vengeance of heaven, lf he do not speak the truth. 1 Leach, 430.
See also
-- Black's Law Dictionary
Legal Definition
Includes every form of attestation by which the party signifies that he is bound in conscience to perform an act faithfully and truthfully. See 59 Minn. 6, 50 Am. St. Rep. 389, 60 N. W. 676.
-- Ballentine's Law Dictionary