Written statements made under oath (a promise to tell the truth) that can be used as evidence in court.
From Medieval Latin 'affidavit' meaning 'he/she has sworn.' The word comes from 'affidare' (to pledge faith). It entered English legal vocabulary in the 15th century.
An affidavit is powerful because the person who signs it is literally saying 'I swear this is true' under penalty of perjury—lying on one gets you in serious legal trouble, which is why courts trust them.
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