The Collaborative International Dictionary of English v.0.48:

Oath \Oath\ ([=o]th), n.; pl. Oaths ([=o][th]z). [OE. othe,
   oth, ath, AS. [=a][eth]; akin to D. eed, OS. [=e][eth], G.
   eid, Icel. ei[eth]r, Sw. ed, Dan. eed, Goth. ai[thorn]s; cf.
   OIr. oeth.]
   1. A solemn affirmation or declaration, made with a reverent
      appeal to God for the truth of what is affirmed. "I have
      an oath in heaven" --Shak.
      [1913 Webster]

            An oath of secrecy for the concealing of those
            [inventions] which we think fit to keep secret.
                                                  --Bacon.
      [1913 Webster]

   2. A solemn affirmation, connected with a sacred object, or
      one regarded as sacred, as the temple, the altar, the
      blood of Abel, the Bible, the Koran, etc.
      [1913 Webster]

   3. (Law) An appeal (in verification of a statement made) to a
      superior sanction, in such a form as exposes the party
      making the appeal to an indictment for perjury if the
      statement be false.
      [1913 Webster]

   4. A careless and blasphemous use of the name of the divine
      Being, or anything divine or sacred, by way of appeal or
      as a profane exclamation or ejaculation; an expression of
      profane swearing. "A terrible oath" --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

oath
    n 1: profane or obscene expression usually of surprise or anger;
         "expletives were deleted" [syn: curse, curse word,
         expletive, oath, swearing, swearword, cuss]
    2: a commitment to tell the truth (especially in a court of
       law); to lie under oath is to become subject to prosecution
       for perjury [syn: oath, swearing]
    3: a solemn promise, usually invoking a divine witness,
       regarding your future acts or behavior; "they took an oath of
       allegiance"

Moby Thesaurus II by Grady Ward, 1.0:

48 Moby Thesaurus words for "oath":
   Bible oath, assurance, avouch, avouchment, avow, avowal, curse,
   cuss, cuss word, dirty name, dirty word, dysphemism, epithet,
   expletive, extrajudicial oath, faith, foul invective, guarantee,
   guaranty, imprecation, ironclad oath, judicial oath, loyalty oath,
   malediction, naughty word, no-no, oath of allegiance,
   oath of office, obscenity, official oath, parole, pledge, plight,
   profane oath, profanity, promise, solemn declaration, solemn oath,
   swear, swearword, sworn statement, test oath, troth, vow, warrant,
   warranty, word, word of honor



The Free On-line Dictionary of Computing (30 December 2018):

OATH

   Object-oriented Abstract Type Hierarchy, a class library for
   C++ from Texas Instruments.


Easton's 1897 Bible Dictionary:

Oath
   a solemn appeal to God, permitted on fitting occasions (Deut.
   6:13; Jer. 4:2), in various forms (Gen. 16:5; 2 Sam. 12:5; Ruth
   1:17; Hos. 4:15; Rom. 1:9), and taken in different ways (Gen.
   14:22; 24:2; 2 Chr. 6:22). God is represented as taking an oath
   (Heb. 6:16-18), so also Christ (Matt. 26:64), and Paul (Rom.
   9:1; Gal. 1:20; Phil. 1:8). The precept, "Swear not at all,"
   refers probably to ordinary conversation between man and man
   (Matt. 5:34,37). But if the words are taken as referring to
   oaths, then their intention may have been to show "that the
   proper state of Christians is to require no oaths; that when
   evil is expelled from among them every yea and nay will be as
   decisive as an oath, every promise as binding as a vow."


Bouvier's Law Dictionary, Revised 6th Ed (1856):

OATH. 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. 10 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:
    Sec. 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.-Sec. 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.-Sec. 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 administer 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.-Sec. 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.-Sec. 5. That the secretary of the senate, and the clerk of the
house of representatives, 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. Sec. 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. Sec. 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 fulfill 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.



The Devil's Dictionary (1881-1906):

OATH, n.  In law, a solemn appeal to the Deity, made binding upon the
conscience by a penalty for perjury.