What is the difference between oath, pledge & vow and where should each word be used? An example should suffice.
To me, the nouns pledge and vow are closer in meaning, to the point of being synonyms. They mean “a solemn promise” (this is NOAD's main definition for both). Oath, while close in meaning, typically has an additional sense of invoking the divine (e.g., an oath taken on the bible). A verb with close meaning to all three is to swear.
In addition to this meaning of “solemn promise”, each of these three nouns has specific additional meanings.
- an oath maybe refer to a profanity or a swear word
- vows maybe have specific meaning of commiting oneself to a given life, e.g. marriage or monastic vows
- pledge has an additional legal meaning, synonymous to surety, bond or guarantee
Before giving you my answer, I'm going to simply state the dictionary definitions of the three words, from the Merriam-Webster Online Dictionary for your information.
Oath: (1) a formal and serious promise to tell the truth or to do something (2)an offensive or rude word that is used to express anger, frustration, surprise, etc.
Pledge: (1) a serious promise or agreement (2) a promise to give money (3)something that you leave with another person as a way to show that you will keep your promise
Vow: (1) a serious promise to do something or to behave in a certain way
Oath, as stated above, can mean either a formal promise or an offensive word. Assuming you are interested in the first definition, we will focus on that. Oath is more commonly used when the person speaking the promise calls upon God to witness to the event, as in the Boy Scout Promise. An oath often comes with a price for not keeping it; this could be death. In my personal opinion, an oath is portrayed as the most 'serious' of the three words.
Pledge is much less formal. Many organizations, such as 4-H, state their moral and ethic affirmations through a pledge. Likewise, the Pledge of Allegiance is a promise of loyalty to one's country. Often, pledge refers to an amount of money someone is agreeing to give to another person.
Vow most often refers to behavior. The person saying their vows is most likely agreeing to act or behave in a certain way for a certain period of time. Marriage vows are the most obvious example, and are intended to be kept for life, as Andrew Leach commented above. The Unbreakable Vow in Harry Potter and the Half-Blood Prince is a good example that illustrates just how serious vows are meant to be.
I hesitate to say this, but I believe that all three could be used somewhat interchangeably within causally writing or speaking. To me, oath has a more solemn tone to it, while pledge clearly rings of patriotism. And who can say the word vow without thinking of a wedding?
Hopefully this cleared things up for you a bit. If you need more examples of usage for each, let me know.
I thought it might be interesting to see how these words were understood and differentiated by reference works over the past hundred years or so. Unfortunately, the coverage of the three terms in synonym dictionaries is somewhat spotty (though still interesting) across the years. Here is what some authorities have said. From James Fernald, English Synonyms and Antonyms (1896):
In the highest sense, as in a court of justice, "an oath is reverent appeal to God in corroboration of what one says," ABBOTT LAW DICT[IONARY]. ...An oath is made to man in the name of God; a vow, to God without the intervention, often without the knowledge, of man. In the lower sense, an oath may be mere blasphemy or profane swearing.
This book doesn't include pledge in the same group of words with oath and vow, but instead lists it with (among other terms) compact, covenant, obligation, pact, promise, and stipulation, under the general heading contract, remarking
All of these words involve at least two parties, tho an engagement or promise may be the act of but one.
Webster's Dictionary of Synonyms (1942) doesn't cover oath or vow, but offers these comments in distinguishing the verb pledge from the verb promise:
Promise, engage, pledge, plight, covenant, contract come into comparison as meaning to give one's word that one will do make, give, accept, or the like, something stipulated. Promise, both as a transitive and as an intransitive verb, implies a giving assurance (usually orally or in writing) but it suggests no further grounds for expectation of the fulfillment of what is promised; [examples omitted]. ... Pledge, chiefly a transitive verb may imply either the giving voluntarily of a promise by some actor words that suggest the giving of a solemn assurance, or the provision of a formal guarantee (as to pledge one's honor that one will see that a dying friends wish is respected; [other examples omitted]), or the putting of another or of others under a solemn promise to do, to forbear, or the like [examples omitted].
The noun forms of pledge associated with the verb forms described here would involve the promise or guarantee or security that the specified action will be performed.
S.I. Hayakawa, Choose the Right Word: A Modern Guide to Synonyms (1968) again passes over oath and vow, but addresses pledge as a noun in some detail:
pledge, bail, bond, collateral, guarantee, security. These words refer to a promise that is backed up in someway and reinforced by the commitment of one's honor or material possessions. Pledge is the most general of these, applying in any case where someone solemnly promises to remain loyal to a principle or to undertake a given task: the Pledge of Allegiance to the Flag; a pledge of $10,000 to the Alumni Fund; a pledge to have the alterations completed by Friday. Only the person's honor backs up his promise in this case.
And finally, Merriam-Webster's Dictionary of Synonyms (1984) examines pledge as a noun in the company of four related nouns:
Pledge, earnest, token, pawn, hostage are comparable when they denote something that is given or held as a sign of another's faith or intention to do what has been promised. Pledge, originally and still in some applications a technical legal term, applies in general to something handed over to another as a token [example omitted], or as security for the performance of an obligation or payment of a debt [examples omitted].
These discussions suggest that though oath, vow, and pledge are all promises, they have different focuses. An oath is a swearing in the presence of God with regard to a commitment one is making to one or more other people (as to tell the truth as a witness in court, or to uphold the dignity of one's office). A vow is a promise to God (or to something philosophically analogous) to accept and discharge faithfully some specified spiritual or material obligation. A pledge is a promise on one's honor or sense of duty to take some action in fulfillment of a contract with other human beings.
The casual interchangeability of these words today is unfortunate, but is understandable in our pluralistic culture. Vows and oaths are more than just simple promises, due to the magnitude of commitment and (perceived?) enforceability. Both vows and oaths can be terminated based on preset conditions, but only the person or entity to whom the oath or vow has been made can release the one who made the oath or vow. Vows also tend to be more permanent and unconditional than oaths because vows imply religious/spiritual conditions that oaths do not. An oath is between one or more humans, usually with God as witness. For example, an oath of office lasts only as long as that term of office. A military officer takes an oath that ends when that officer is discharged or separated from the service. The oath is enforceable by law, but only during the term of enlistment, warrant, or commission, and within other limitations of the law. A vow is between Man and God, usually with other humans as witnesses. For example, a marriage vow is actually a personal commitment between each individual spouse and God, and not simply an emotional promise said by each spouse to the another in the happiness of a wedding ceremony. In this sense, if one spouse violates their vow, the other is not necessarily off the hook, especially if their vow was "... for better or for worse, until death..." The distinction is important today, as we tend to treat marriage vows more like simple verbal contracts, especially when they are violated. However, in order to speak a vow in good faith, one must believe that God is ultimately the enforcer of the vow, not just some earthly power.