For comprehensive definitions of legal terms, you may wish to consult a legal dictionary. An affidavit is a sworn statement of fact that can be used in a variety of legal proceedings, from bankruptcy cases to family and civil litigation matters.Affirmation: A formal declaration that a statement is true. To lie is perjury and a person can be punished. A written declaration upon oath made before an authorized official. An affidavit is a statement made under oath, claiming that a fact or set of facts is true to the best of the "affiant's" knowledge. Find definitions of legal terms to help understand the federal court system.