Blackmail is an unlawful demand of money or property under threat or coercion to cause bodily harm, or to damage property, or to accuse of a crime, or to expose disgraceful defects. It is similar to extortion.
The word is derived from the word for tribute paid by English border dwellers to Scottish chieftains in return for immunity from raids. Such 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).
Blackmail is also the name of Alfred Hitchcock's first talkie film and the name of a german band.