A counterfeit is a fake or a forgery. If you painted an uncanny copy of the "Mona Lisa" and tried to pass it off as the original, you'd have a counterfeit on your hands.
An exact imitation of anything — a work of art, a ten dollar bill — is a counterfeit, and the act of creating the fake copy is the verb to counterfeit. You can also describe the forgery using the adjective form of counterfeit: a counterfeit Rolex watch. In Old French, the word contre, "counter or against", together with the word faire, "to make," joined together to mean "to make in imitation," which led to our counterfeit.