Affidavit definition: . See examples of AFFIDAVIT used in a sentence.It is a legal document detailing knowledge the person has or facts as they know them to be. Learn more in our legal dictionary. 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. An affidavit is a written statement, sworn to be true, that can be used as evidence in legal proceedings. A written statement that someone makes after promising officially to tell the truth. An affidavit can be used as proof in a law court. The process server must fill out and sign the proof of service form, which shows how, when, and where the papers were served, and who received them.