Affidavit
A written statement of facts sworn under oath before a notary or other authorized officer.
An affidavit is a voluntary written statement of facts, sworn or affirmed under oath before an officer authorized to administer oaths (such as a notary). Affidavits support motions, applications (like the AO-240 financial affidavit), and evidentiary showings.
A related instrument, the unsworn declaration under 28 U.S.C. § 1746, carries the same effect in federal practice when made "under penalty of perjury."