To counterfeit means to imitate something authentic, with the intent to steal, destroy, or replace the original, for use in illegal transactions, or otherwise to deceive individuals into believing that the fake is of equal or greater value than the real thing. Counterfeit products are fakes or unauthorized 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 items such as clothing, handbags, shoes, pharmaceuticals, automobile parts, unapproved aircraft parts (which have caused many accidents), watches, electronics and electronic parts, software, works of art, toys, and movies.

Counterfeit products tend to have fake company logos and brands, which results in patent or trademark infringement in the case of goods. They also have a reputation for being lower quality, sometimes not working at all, and sometimes containing toxic substances such as lead. 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, mostly paper money, is usually attacked aggressively by governments worldwide.