Blackmail is the crime of threatening to reveal embarrassing, disgraceful or damaging information about a person to the public, family, spouse or associates unless money is paid to purchase silence. It is a form of extortion. Because the information is usually substantially true, it is not revealing the information that is criminal, but demanding money to withhold it.
The term was originally used for the payment levied upon the farmers of the Scottish border before the 18th century to provide immunity from raids by Scottish bands. If the farmers didn't pay the mail (an old term for rent), the chieftains would steal their cattle and crops. Since this was considered evil, it was considered "black".
Blackmail has been defined in the broad sense to mean "compelling someone to act against their will or gaining or attempting to gain something of value." Courts vary on interpreting what "something of value" includes, but it is not necessarily a money payment in all cases.
State laws vary, but the following is an example of a state blackmail statute:
Blackmail is gaining or attempting to gain anything of value or compelling another to act against such person's will, by threatening to communicate accusations or statements about any person that would subject such person or any other person to public ridicule, contempt or degradation.
Blackmail is a severity level 7, nonperson felony"