A counterfeit is a fake or unauthorized replica of a genuine product, such as money, documents, designer items, or other valuable goods. Counterfeiting generally involves creating an imitation of a genuine item that closely resembles the original to deceive others into believing it is authentic.

Counterfeit products are often made to take advantage of the higher value of the original product, typically using lower-quality materials or production methods. Counterfeit food, drinks, medicines, and personal care products can contain harmful or inactive ingredients, causing anything from mild issues to serious, life-threatening ones. Counterfeit footwear, clothing, and accessories have been found to contain high levels of lead, arsenic, and phthalates.

Counterfeit money is currency that is produced without the legal sanction of the state or government; this is a crime in all jurisdictions of the world. The United States Secret Service, mostly known for its guarding-of-officials task, was initially organized primarily to combat the counterfeiting of U.S. dollars in the wake of the American Civil War. Both sides had printed counterfeit notes in attempts to destabilize the other's economy, an example of economic warfare.

Counterfeit government bonds are public debt instruments that are produced without legal sanction, with the intention of "cashing them in" for authentic currency or using them as collateral to secure loans or lines of credit through legitimate channels.

Forgery is the process of making facsimiles or adapting documents with the intention to deceive. It is a form of fraud, and is often a key technique in the execution of identity theft. Uttering is a term in United States law for the forgery of non-official documents, such as a trucking company's time and weight logs.

Questioned document examination is a scientific process for investigating many aspects of various documents, and is often used to examine the provenance and verity of a suspected forgery. Security printing is a printing industry specialty, focused on creating legal documents which are difficult to forge.

Counterfeit goods are products that illegally use a registered trademark on items identical or similar to the original, designed to mislead buyers into thinking they are purchasing authentic products. Similarly, pirated goods infringe copyright, typically involving unauthorized reproductions or distributions of copyrighted works, such as movies, music, or software. The exact definitions of both counterfeit and pirated goods can vary by country, depending on local intellectual property laws.

The colloquial terms "dupe" (short for "duplicate") or "knockoff" are often used interchangeably with "counterfeit," though they have different meanings. Dupes and knockoffs generally mimic the physical look of other products without copying the brand name or logo of a trademarked item, unlike counterfeits. Despite this distinction, counterfeits are frequently misleadingly marketed as dupes.