What is Blackmail?

Legal Definition
Blackmail is an act, often a crime, involving unjustified threats to make a gain (commonly money or property) or cause loss to another unless a demand is met. Essentially, it is coercion involving threats to reveal substantially true or false information about a person to the public, a family member, or associates, or threats of physical harm or criminal prosecution. It is the name of a statutory offence in the United States, England and Wales, Northern Ireland, Victoria, Australia, and Tasmania, and has been used as a convenient way of referring to other offences, but was not a term of art in English law before 1968. It originally meant payments rendered by settlers in the Counties of England bordering Scotland to chieftains and the like in the Scottish Lowlands, in exchange for protection from Scottish thieves and marauders into England.

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. Some US states distinguish the offenses by requiring that blackmail be in writing. In some jurisdictions, the offence of blackmail is often carried out during the act of robbery. This occurs when an offender makes a threat of immediate violence towards someone in order to make a gain as part of a theft. For example, the threat of "Your money, or your life!" is an unlawful threat of violence in order to gain property.
-- Wikipedia
Legal Definition
Coercion by unjustifiably threatening to reveal to another person or to the public substantially true information that is embarrassing, injurious, or incriminating. Although revealing this information is not in itself a crime, blackmail that comes in the form of demanding money in exchange for withholding information is considered criminal behavior.
Legal Definition
term for extortion and it is a criminal act where a person will attempt to get money from another person by threats.
Legal Definition
When rents were reserved payable in work, grain, and the like, they were called reditus nigri, or black mail, to distinguish them from white rents or blanch farms, or such as were paid in money. Vide Alba firma.
-- Bouviers Law Dictionary
Legal Definition
1. In one of its original meanings, this term denoted a tribute paid by English dwellers along the Scottish border to influential chieftains of Scotland, as a condition of securing immunity from raids of marauders and border thieves.

2. It also designated rents payable in cattle, grain, work, and the like. Such rents were called "black-mail," (reditus nigri,) in distinction from white rents, (blanche firmes,) which were rents paid in silver.

3. The extortion of money by threats or overtures towards criminal prosecution or the destruction of a man's reputation or serial standing. In common parlance, the term is equivalent to, and synonymous with, "extortion,"—the exaction of money, either for the performance of a duty, the prevention of an injury, or the exereise of an influence. It supposes the service to be unlawful, and the payment involuntary. Not infrequently it is extorted by threats, or by operating upon the fears or the credulity, or by promises to conceal, or offers to expose, the weaknesses, the follies, or the crimes of the victim. Bdsall v. Brooks, 3 Rob. (N. Y.) 284, 17 Abb. Prac. 221; Life Ass'n v. Boogher, 3 Mo. App. 173 ; Hess v. Sparks, 44 Kan. 465, 24 Pac. 979, 21 Am. St. Rep. 300; People v. Thompson, 97 N. Y. 313; Utterback v. State, 153 Ind. 545, 55 N. El 420; Mitchell v. Sharon (C. C.) 51 Fed. 424.
-- Black's Law Dictionary
Legal Definition
Tribute levied by marauders who operated on the Scotch border.
-- Ballentine's Law Dictionary