oath
noun [ C ]
uk/əʊθ/ usan official promise, especially when you promise to tell the truth in court:
The witness took the oath.
A psychologist cannot be compelled to attend a disciplinary hearing or to swear on oath.
on/under oath
LAW
having officially promised to tell the truth in court:
She was called to testify under oath before a congressional panel.
take the oath of office
GOVERNMENT
to accept a job in a government, especially in the US, in a ceremony which requires you to make an official promise to serve your country:
The newly-elected Democrat took the oath of office in the Senate yesterday and quickly introduced his first bill.