An affidavit is a written statement. A person, known as the affiant, swears under oath to the statement's truthfulness.Adverse possession - Claim of ownership of property based upon open and hostile possession and occupancy. Affiant - A person who has made an affidavit. This guide provides information on how to reference using the Chicago Footnote referencing style. An affidavit is a written statement made under oath that is typically used in legal proceedings. Learn their purpose, when to use, and how to write. Affidavit definition: . See examples of AFFIDAVIT used in a sentence. What does the noun affidavit mean?