It explains commonly used legal terms, which can help everyone better understand legal papers from indictment through sentencing. An affidavit is a written declaration under oath, made without notice to the adverse party.An affidavit is a written statement made under oath that is typically used in legal proceedings. Learn their purpose, when to use, and how to write. A written statement that someone makes after promising officially to tell the truth. An affidavit can be used as proof in a law court.