Glossary
Affidavit
A written, signed statement made under oath, witnessed by a notary or other officer authorized to administer oaths. Affidavits are commonly used to support motions instead of live testimony.
Affidavits substitute for live testimony in many contexts. Instead of bringing a witness into court, you submit their sworn written statement. Courts use affidavits routinely on motions, default-judgment proceedings, and other situations where the issue can be decided on papers.
A "declaration under penalty of perjury" is the modern equivalent that doesn't require notarization in most contexts (federal court, for example, uses 28 U.S.C. ยง 1746 declarations). The legal effect is the same: false statements can lead to perjury charges.