Affirmation: A formal declaration that a statement is true. To lie is perjury and a person can be punished.Affiant - Person who prepares and signs an affidavit. Affidavit - Written statement made under oath. An affidavit is a written statement that you swear is true and that may be used as evidence in a court of law. Find definitions and American Sign Language translations for legal terminology used in legal settings. Affidavit definition: A written declaration made under oath before a notary public or other authorized officer. There is one meaning in OED's entry for the noun affidavit. A legal document in which a person provides evidence of certain facts and events in writing. The courts are not to fill in the gaps in legislation.