a solemn sworn promise that binds the one who makes it, calling God to witness that the word will be kept
An oath in the Bible is far weightier than a casual promise. To swear was to put oneself under God's own eye, staking one's standing before him on doing exactly what was said. Because the word rests so heavily on the person who speaks it, one oath can be viewed from three angles that all share this single core: the promise a person makes, the promise God makes, and the oath whose binding force is a curse.
Most often the word names a human pledge. It settles a dispute, it seals a private agreement (Abraham binds his servant, the spies bind Rahab), and it holds a whole community to a course of action (Israel spares the Gibeonites because it had sworn to). To break such an oath was not merely to disappoint someone but to lie in God's presence, so an oath, once given, was treated as unbreakable even when keeping it proved costly.
The same word describes the promise God himself makes. When Scripture speaks of the oath God swore to Abraham, Isaac, and Jacob, it means God has bound his own faithfulness to his word, giving his people the strongest possible ground for trusting him. Here the oath is not a risk the swearer takes but a gift the hearer receives.
Because an oath usually spelled out what should happen if it were broken, the word can also point to the curse itself. In the ritual for a wife suspected of unfaithfulness, in the covenant Israel entered to obey God's law, and where a person's very name becomes a word others use when they curse, the oath and its threatened penalty run together as one idea.