Counterfeiting is an act of creating an imitation of an item with the intent of passing it off as the real thing. It often involves fake currency. Such activity is generally considered a crime.
Harry Mudd's police record in 2266 included his purchase of a space vessel with counterfeit currency. (TOS: "Mudd's Women")
Many unscrupulous individuals produced fabrications of the Shroud of the Sword over the years, claiming they were authentic. (DS9: "The Sword of Kahless")