What is Counterfeit?

Legal Definition
To counterfeit means to imitate something. Counterfeit products are fakes or unauthorised replicas of the real product. Counterfeit products are often produced with the intent to take advantage of the superior value of the imitated product. The word counterfeit frequently describes both the forgeries of currency and documents, as well as the imitations of clothing, handbags, shoes, pharmaceuticals, aviation and automobile parts, watches, electronics (both parts and finished products), software, works of art, toys, movies and any other type of products such as those. Counterfeit products tend to have fake company logos and brands. In the case of goods, it results in patent infringement and/or trademark infringement. Counterfeit consumer products have a reputation for being lower quality (sometimes not working at all) and may even include toxic elements. This has resulted in the deaths of hundreds of thousands of people, due to automobile and aviation accidents, poisoning, or ceasing to take essential compounds (e.g. in the case a person takes non-working medicine).

The counterfeiting of money is usually attacked aggressively by governments worldwide. The counterfeiting of goods is condoned by some governments. Paper money is the most popular product counterfeited.
-- Wikipedia
Legal Definition
Criminal law. To make something false, in the semblance of that which is true; it always implies a fraudulent intent. Vide Vin. Ab. h. t. Forgery.
-- Bouviers Law Dictionary
Legal Definition
In criminal law. To forge; to copy or imitate, without authority or right, and with a view to deceive or defraud, by passing the copy or thing forged for that which is original or genuine. Most commonly applied to the fraudulent and criminal imitation of money. State v. McKenzie, 42 Me. 392; U. S. v. Barrett (D. C.) Ill Fed. 369; State v. Calvin, R. M. Charlt (Ga.) 159; Mattison v. State, 3 Mo. 421.
See also
-- Black's Law Dictionary
Legal Definition
A fraudulent imitation of a genuine article. See 42 Me. 392.
-- Ballentine's Law Dictionary