Forgery is a white-collar crime that generally refers to the false making or material alteration of a legal instrument with the specific intent to defraud. Tampering with a certain legal instrument may be forbidden by law in some jurisdictions but such an offense is not related to forgery unless the tampered legal instrument was actually used in the course of the crime to defraud another person or entity. Copies, studio replicas, and reproductions are not considered forgeries, though they may later become forgeries through knowing and willful misrepresentations.

Forging money or currency is more often called counterfeiting. But consumer goods may also be counterfeits if they are not manufactured or produced by the designated manufacturer or producer given on the label or flagged by the trademark symbol. When the object forged is a record or document it is often called a false document.

This usage of "forgery" does not derive from metalwork done at a blacksmith's forge, but it has a parallel history. A sense of "to counterfeit" is already in the Anglo-French verb forger, meaning "falsify".

A forgery is essentially concerned with a produced or altered object. Where the prime concern of a forgery is less focused on the object itself – what it is worth or what it "proves" – than on a tacit statement of criticism that is revealed by the reactions the object provokes in others, then the larger process is a hoax. In a hoax, a rumor or a genuine object planted in a concocted situation, may substitute for a forged physical object.

The similar crime of fraud is the crime of deceiving another, including through the use of objects obtained through forgery. Forgery is one of the techniques of fraud, including identity theft. Forgery is one of the threats addressed by security engineering.

In the 16th century, imitators of Albrecht Dürer's style of printmaking improved the market for their own prints by signing them "AD", making them forgeries. In the 20th century the art market made forgeries highly profitable. There are widespread forgeries of especially valued artists, such as drawings originally by Pablo Picasso, Paul Klee, and Henri Matisse.

A special case of double forgery is the forging of Vermeer's paintings by Han van Meegeren, and in its turn the forging of Van Meegeren's work by his son Jacques van Meegeren.