USCIS.gov now has an online dictionary (glossary). You can use this dictionary to quickly look up a definition or explanation for a topic.An affidavit is a written statement made under oath or affirmation. It's used as evidence in legal proceedings. Interviewer may fill out the application. Affidavit definition: . See examples of AFFIDAVIT used in a sentence. Completing and signing Form I-864 makes you the sponsor. The term and definition are singled out in a dedicated section of organizational material. b. An affidavit is a written statement that you swear is true and that may be used as evidence in a court of law.