It comes from a past tense form of the Latin verb affidare, meaning "to pledge"; in Latin, affidavit translates to "he or she has made a pledge. It's a written statement that an individual, known as the 'affiant,' makes willingly and truthfully under an oath or affirmation.An affidavit is a document which is between the petitioner and the immigrant visa applicant. 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. 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. The official website of the Federal Trade Commission, protecting America's consumers for over 100 years.