What does Affidavit mean? An affidavit is a written declaration under oath, made without notice to the adverse party.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. A written declaration upon oath made before an authorized official. An affidavit is a written statement made under oath or affirmation. Constitute a complete affidavit. (Law) A sworn statement in writing; a declaration in writing, signed and made upon oath before an authorized magistrate.