An affidavit is a sworn statement a person makes before a notary or officer of the court outside of the court asserting that certain facts are true to the best of that person’s knowledge. Affidavits by both plaintiff/prosecution and defense witnesses are usually collected in preparation for a trial.

In the event a witness’s testimony contradicts what they stated in their affidavit, the affidavit can be introduced as evidence to impeach that witness in accordance with Federal Rule of Evidence 607. An impeachment of this sort can potentially lead to criminal charges of perjury if the contradiction was made knowingly, intentionally, and involved a material issue.