Evidence which can be legally and properly introduced in a trial. Adversary proceeding.An affidavit is a sworn statement that is in writing. Affidavits are usually used in a court or in negotiations. In the legal sense, the document a plaintiff files with the court which contains allegations and damages sought. A complaint generally starts a lawsuit. 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. The FindLaw Legal Dictionary -- free access to over 8260 definitions of legal terms. Search for a definition or browse our legal glossaries.