What word is a synonym for oath?

Here are some synonyms for oath, depending on the context:

Formal:

* Vow: A solemn promise

* Pledge: A promise to do something

* Swear: To make a solemn declaration

* Affirmation: A formal statement of truth

* Declaration: A formal statement, often in a court of law

Informal:

* Promise: A commitment to do something

* Word: A statement that is considered binding

* Guarantee: A promise that something will happen

The best synonym for oath will depend on the specific situation and the intended meaning.