A written declaration upon oath made before an authorized official. Learn more in our legal dictionary.An affidavit is a written statement, sworn to be true, that can be used as evidence in legal proceedings. Find definitions of legal terms to help understand the federal court system. This reporter's guide is not a comprehensive legal dictionary or manual. Rather it includes words and concepts that reporters most often misunderstand. An affidavit is a self-serving declaration used in legal service. Consider it sort of like a confession, or a witness statement.