Either a statement of fact or a promise, usually made by a person while holding a sacred book in his or her right hand, that a person will tell the truth. For those who conscientiously object to making a sacred oath, an affirmation is given instead.

Any matter of fact that a party to a claim or action offers to provide to prove or disprove an issue in the case. The strongest type of evidence is that which provides direct proof of the truth of an assertion. At the other extreme is evidence that is merely consistent with an assertion but does not rule out other, contradictory assertions, and may be viewed as circumstantial evidence.