Affidavit definition: . Learn more in our legal dictionary.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. 'Affidavit' in a sentence: We have a signed affidavit stating that the two men were seen entering the building. See more meanings of 'AFFIDAVIT' with examples. An affidavit is a written statement that you swear is true and that may be used as evidence in a court of law. An affidavit is written form of what has happened that someone has witnessed. A jury verdict that a criminal defendant is not guilty, or the finding of a judge that the evidence is insufficient to support a conviction. An affidavit is the written equivalent of verballygiven court testimony.