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. We hope that you find this Handbook helpful, and that it provides some aid in protecting your rights behind bars. What is an affidavit? An affidavit essentially is written testimony. Affidavit definition: . See examples of AFFIDAVIT used in a sentence. A lot during the immigration process. What is an affidavit?