Affidavit

Etymology

A Medieval Latin verb pressed into service as a noun: “affidavit” means “he has pledged his faith,” from “affidare” (to trust), from “ad-” plus “fides” (faith) — the same root as “fidelity” and “confide.” The document is named for its opening Latin verb: an oath frozen into a title.

noun · /af-uh-DAY-vit/
A written statement of facts sworn to be true under oath, usable as evidence in court.
What it names
The paperwork version of putting your hand on the Bible — reach for it to name a sworn statement made on paper rather than live in the witness box.
Example
She signed an affidavit before a notary swearing she had witnessed the accident.
Related idea
An affidavit is sworn evidence in writing; testimony is sworn evidence given live and subject to cross-examination — which is why courts often trust live testimony over a signed affidavit.