oath n
1: a solemn attestation of the truth of one's words or the sincerity of one's intentions; specif: one accompanied by calling upon a deity as a witness
2: a promise (as to perform official duties faithfully) corroborated by an oath compare perjury
under oath: under a solemn and esp. legal obligation to tell the truth (as when testifying)
Merriam-Webster’s Dictionary of Law. Merriam-Webster. 1996.