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).