Traditionally an oath (from Anglo-Saxon āð, also called plight) is either a statement of fact or a promise taken by a sacrality 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 is 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.
The word come from Anglo-Saxon āð judicial swearing, solemn appeal to deity in witness of truth or a promise," from Proto-Germanic *aiþaz (source also of Old Norse eiðr, Swedish ed, Old Saxon, Old Frisian eth, Middle Dutch eet, Dutch eed, German Eid, Gothic aiþs "oath"), from PIE *oi-to- "an oath" (source also of Old Irish oeth "oath"). Common to Celtic and Germanic, possibly a loan-word from one to the other, but the history is obscure and it may ultimately be non-Indo-European, in reference to careless invocations of divinity, from the late 12th century.
Oaths usually have referred to a deity significant in the cultural sphere in question. The reciter's personal views upon the divinity of the aspects considered sacred in a predictated text of an oath may or may not be taken in to account. There might not be alternative personal proclamations with no mention of the sacred dogma in question, such as affirmations, to be made. This might mean an impasse to those with unwillingness to edify the dogma they see as untrue and those who decline to refer to sacred matters on the subject at hand.
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 his or her 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.