oathoath /əʊθoʊθ/ noun [countableC]1a formal promise to do somethingThe president has taken an oath to support and defend the Constitution.
The new president will take the oath of office next week.
2be under oath (also be on oath British EnglishBrE)LAW to have made a formal promise to tell the truth in a court of lawThe defendant acknowledged that he lied under oath.
The witnesses should be examined on oath.
3take the oath to make a formal promise to tell the truth in a court of law