Blackmail is an act of coercion using the threat of revealing or publicizing either substantially true or false information about a person or people unless certain demands are met. It is often damaging information, and it may be revealed to family members or associates rather than to the general public. These acts can also involve using threats of physical, mental or emotional harm, or of criminal prosecution, against the victim or someone close to the victim. It is normally carried out for personal gain, most commonly of position, money, or property.

Blackmail may also be considered a form of extortion. Although the two are generally synonymous, extortion is the taking of personal property by threat of future harm. Blackmail is the use of threat to prevent another from engaging in a lawful occupation and writing libelous letters or letters that provoke a breach of the peace, as well as use of intimidation for purposes of collecting an unpaid debt.

In many jurisdictions, blackmail is a statutory offense, often criminal, carrying punitive sanctions for convicted perpetrators. Blackmail is the name of a statutory offense in the United States, England and Wales, and Australia, and has been used as a convenient way of referring to certain other offenses, but was not a term used in English law until 1968.

Blackmail was originally a term from the Scottish Borders meaning payments rendered in exchange for protection from thieves and marauders. The "mail" part of blackmail derives from Middle English male meaning "rent or tribute". This tribute (male or reditus) was paid in goods or labour ("nigri"); hence reditus nigri, or "blackmail". Alternatively, it may be derived from two Scottish Gaelic words blathaich - to protect; and mal - tribute or payment.