Forgery, deceive or injure another, he falsely makes, completes or alters a written instrument which is or purports to be. "written instrument," "falsely made," "falsely completed," and.Under New York Law, a "forged instrument" means a written instrument which has been falsely made, completed or altered. Criminal Possession of a Forged Instrument in the Third Degree is a Class A misdemeanor. In legal terms, a forged instrument is a document or item that has been unlawfully altered or created with the intent to deceive or defraud. Forgery is a white collar criminal offense that involves creating a false written document with the intent to defraud.