term:

Oath

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)

Source: Merriam-Webster's Dictionary of Law ©1996. Merriam-Webster, Incorporated. Published under license with Merriam-Webster, Incorporated.