Oath (noun)

1. A solemn promise, often invoking a divine witness, regarding one's future action or behavior.
2. A formal statement of the truth of a statement or the authenticity of a document.

Origin:

From old english āð, from proto-germanic *aþaz, from proto-indo-european *h₁óitos.

Examples:

1. He took an oath of allegiance to the country.
2. She took an oath to tell the truth in court.
3. The judge administered the oath to the witness.
4. The soldier took an oath to defend his country.
5. The employee took an oath of confidentiality.