A sworn statement in writing made especially under oath or on affirmation (see affirmation sense 2) before an authorized magistrate or officer. Affidavit definition: .See examples of AFFIDAVIT used in a sentence. Affidavit: A written declaration made under oath before a notary public or other authorized officer. An affidavit is a legal document that takes the place of oral testimony in legal proceedings. An affidavit is a written statement made under oath or affirmation. 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. Affidavit definition: A written declaration made under oath before a notary public or other authorized officer.