wikipedia:
Blackmail is an act of coercion using the threat of revealing or publicizing either substantially true or false, and often damaging, information about a person, to the public, family members, or associates unless certain demands are met. It may involve using threats of physical, mental or emotional harm, or of criminal prosecution, against the victim or someone or close to the victim.[1][2] It is normally carried out for personal gain, most commonly of position, money, or property.
Cambridge dictionary:
the act of threatening to harm someone or someone's reputation unless the person does as you say, or a payment made to someone who has threatened to harm you or your reputation if you fail to pay the person
uslegal.com
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.
and even the Urban Dictionary:
To force (someone) to do something by using threats or manipulating their feelings.