Blackmail is the crime of threatening to reveal substantially true information about a person to the public, a family member, or associates unless a demand made upon the victim is met. This information is usually of an embarrassing and/or socially damaging nature. As the information is substantially true, the act of revealing the information may not be criminal in its own right nor amount to a civil law defamation; the crime is making demands to withhold it.

Blackmail is similar to extortion—the difference being that extortion involves an underlying, independent criminal act, while blackmail does not.

The word is derived from the word for tribute paid by English and Scottish border dwellers to Border Reivers in return for immunity from raids. This tribute was paid in goods or labour (reditus nigri, or "blackmail"): the opposite is blanche firmes or reditus albi, or "white rent" (denoting payment by silver).

English law

Under section21(1) of the Theft Act 1968 of English law, a person commits the offence:

if, with a view to gain for himself or another or with intent to cause loss to another, he makes any unwarranted demand with menaces; and for this purpose a demand with menaces is unwarranted unless the person making it does so in the belief:

(a) that he has reasonable grounds for making the demand; and

(b) that the use of the menaces is a proper means of reinforcing the demand.

The Act uses the word "menaces" which is considered wider in scope than "threat" and involves a warning of any consequences known to be considered unpleasant by the intended victim. This covers the spectrum from actual or threatened violence to the victim or others, through damage to property, to the disclosure of information.

Pretexts for blackmail have included the threat to reveal adultery or criminal acts. But whatever the nature of the menace, it must be direct. Any vague threat to cause "something bad" to happen to some other person, except when certain demands are met, should not affect the mind of an ordinary person.

Commercial blackmail has become more common. This arises when a large commercial organisation receives credible information that it will suffer loss or damage in a particular way unless money is transferred. There are two major areas of threat:

• denial of service attacks target corporations that have a major presence on the internet. Disrupting the portal through which on-line sales are made could seriously affect the corporation's revenue flow and demonstrating an ability to orchestrate consistent attacks may well represent a sufficient menace for these purposes; and
• introducing poisons or other dangerous chemicals into the products offered for sale in a supermarket or other large store could significantly damage retail sales, or influence a manufacturer or national distributor. For example, a blackmailer threatened Masterfoods Corporation, the company that manufactures Mars Bars in Australia, claiming to have poisoned seven Mars and Snickers bars at random in New South Wales.