A solemn pledge by someone that a statement is true or that written documents are true/correct.
Merriam-Webster Online Dictionary
oath (noun)1.
a) (1) a solemn usually formal calling upon God or a god to witness to the truth of what one says or to witness that one sincerely intends to do what one says
(2) a solemn attestation of the truth or inviolability of one's words
b) something (as a promise) corroborated by an oath
2.
an irreverent or careless use of a sacred name , broadly - swearword
Merriam-Webster Online Thesaurus
oath (noun)a person's solemn declaration that he or she will do or not do something
SYNONYMS:
oath, pledge, troth, vow, wordRELATED WORDS:
appointment, arrangement, commitment, engagement, obligation; agreement, compact, contract, covenant; assurance, guarantee, guaranty, undertaking; bail, bond, deposit, gage, pawn, security, token, warranty