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.Must be at least 18 years old. 2. 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 written statement made under oath or affirmation. J), two Appellant affidavits (Exh. Affidavit definition: . See examples of AFFIDAVIT used in a sentence.