Counterfeit: An imitation of a document, product or its packaging that is made with the intent to deceptively represent the item as the genuine article.

According to the trade related aspects of the Intellectual Property Rights (The trips agreement), “counterfeit trademark goods” shall mean any goods, including packaging, bearing without authorization a trademark which is identical to the trademark validly registered in respect of such goods, or which cannot be distinguished in its essential aspects from such a trademark, and which thereby infringes the rights of the owner of the trademark in question under the law of the country of importation”

In layman language, the making of an imitation, copy of forgery of a genuine document, card, product, label or package with the intention to deceive or defraud is counterfeiting.

According to the Indian Penal Code, a person is called a counterfeiter when he is she causes one thing to resemble another, intending by means of their resemblance to practice deception or knowing it to be likely that the deception will be practical.

Serialization implementation has a few components:

It is important to identify the type of counterfeiting threat prior to selecting or developing effective countermeasures.

Types of counterfeiting (Adapted from (Spink, 2009b , Spink, 2007 ))
Adulterate: A component of the legitimate finished product is fraudulent
Tamper: Legitimate product and package are used in a fraudulent way
Over-run: Legitimate product is made in excess of production agreements
Theft: Legitimate product is stolen and passed off as legitimately procured
Diversion: The sale or distribution of legitimate product outside of intended markets
Simulation: Illegitimate product is designed to look like but not exactly copy the legitimate product
Counterfeit: All aspects of the fraudulent product and package are fully replicated
Note: In each case, fraudsters may not be following the regulatory definitions of Good Manufacturing Practices (GMPs), Good Agricultural Practices (GAPs), or Good Hygiene Practices (GHPs).
Another important distinction for each type of product counterfeiting is that products could be deceptive or non-deceptive. Deceptive counterfeit products are presented in the marketplace as being genuine with the intent to deceive the purchaser. Non-deceptive counterfeit products are presented in the marketplace as counterfeit or fraudulent with no intent to deceive the purchaser (for more information see (OECD, 2007a) or (Spink, 2011)). Non-deceptive counterfeit products are marketed to consumers who seek counterfeit products such as apparel and luxury goods. Effective countermeasures must evaluate whether consumers are intending to buy genuine or counterfeit products.
From https://crimesciencejournal.springeropen.com/articles/10.1186/2193-7680-2-8

The spread of counterfeiting is far and wide, and ranges from counterfeit automobile and aerospace parts to fake luxury items. The trade in counterfeited goods is worth a whopping $462 billion (£321bn) a year, according to the most recent figures from the OECD and the EU’s Intellectual Property Office, with a 2017 report predicting it could hit an astonishing $2.3 trillion (£1.7 trillion) by 2022. As per OECD, the top 10 most counterfeited goods in 2016 include;

Footwear
Clothing
Electrical Machinery
Leather Articles
Watches
Instruments, optical and medicals
Perfumes and cosmetics
Toys
Pharmaceutical
Jewellery

In past, luxury items tend to be the most counterfeited products because they are more valuable, but, today the list is not limited to any specific products as its is available in almost all sectors including most of the day to day items used such as automotive components, tobacco, FMCG products etc. etc.

Counterfeiting activities are heavenly bleeding all the sectors. While many counterfeit products only cause economic losses to brands and authorities, there are many reported cases of counterfeit goods causing health and safety problems, which, in some cases, have led to serious injuries and / or death – especially in products related to baby food, pharmaceuticals and automotive parts.