An oath (from Anglo-Saxon āð, also called plight) is either a promise or a statement of fact calling upon something or someone that the oath maker considers sacred, usually 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, to make a solemn vow.

The essence of a divine oath is an invocation of divine agency to be a guarantor of the oath taker's own honesty and integrity in the matter under question. By implication, this invokes divine displeasure if the oath taker fails in their sworn duties. It therefore implies greater care than usual in the act of the performance of one's duty, such as in testimony to the facts of the matter in a court of law.

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.