A witness whose legal statement is contained in an affidavit or sworn deposition: "An affiant is someone who makes a declaration in writing which is made with an oath before someone who is authorized to administer oaths; especially, for use as evidence in a court of law."

"James was an affiant who gave testimony to the police about the driver of the car that ran into the back of another car and he swore to what he said to an officer of the court and signed the statement."