Blackmail is the crime of threatening to reveal damaging or embarrassing information in order to coerce money or other goods or forms of cooperation out of the victim. For blackmail to be effective, the blackmailer must, in most cases, have physical proof of the information he or she threatens to reveal, such as photographs or letters. Blackmail is often considered synonymous with extortion, and in this sense it may rely on a threat of action other than exposing the victim's secrets.

Some laws distinguish between blackmail and extortion, while others do not. Blackmail may be defined as extortion attempts in writing. Alternatively, blackmail may refer only to threats of action that is not illegal per se, such as revealing compromising photographs, while extortion relies on more active threats, such as physical harm.

The victim of blackmail is typically threatened with exposure of his or her private life, the consequences of which can range from embarrassing to socially devastating to legally damning. A blackmailer may threaten to expose the victim's extramarital affair, for example. Homosexuals were often blackmailed in the past, though this is less common as alternative sexualities are increasingly more accepted. At its most serious, blackmail may rest on the exposure of a serious crime, which would do infinitely more damage to the victim than complying with the blackmailer. Even secret information that is not of a criminal nature, however, can make the victim of blackmail feel that he or she has no recourse against the crime.

A relatively new form of blackmail, more similar to extortion, is known as commercial blackmail. In this crime, a business is the victim. The blackmailer threatens an action which would be devastating to the company's sales or reputation and typically demands a large payment. The perpetrator may, for example, threaten to interfere with the company's ability to conduct Internet sales. In a recent case of commercial blackmail in Australia, the blackmailer claimed to have poisoned a small random selection of the victim's candy bar products.