Affidavit definition: A written declaration made under oath before a notary public or other authorized officer. A written statement that someone makes after promising officially to tell the truth.An affidavit can be used as proof in a law court. 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. 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. An affidavit is a written statement that you swear is true and that may be used as evidence in a court of law. In a nutshell, an affidavit is a sworn statement that is documented in writing. Affidavit filed with motion for summary judgment. Fed.