This is evidence that does not forget. It is not confused by the excitement of the moment. It is not absent because human witnesses are. It is factual evidence. Physical evidence cannot be wrong; it cannot perjure itself; it cannot be wholly absent. Only its interpretation can err.
Oath definition, a solemn appeal to a deity, or to some revered person or thing, to witness one's determination to speak the truth, to keep a promise, etc.: to testify upon oath. See more. Dictionary.com
Under the laws of a state it may be considered treason or a high crime to betray a sworn oath of office. The word 'oath' and the phrase 'I swear' refer to a solemn vow. For those who choose not to, the alternative terms 'solemn promise' and 'I promise' are sometimes used.