The act of using previously attained or recently attained information, to make the victim do what you want or else you reveal this information that would affect them.
Lady : " I saw you having sex with Sheryl."
Guy : "What?! You did?!"
Lady : "Yep, So unless you have sex with me too, I'll tell."
- Lady is Fugly"
Guy : " You can't Black Mail me!"
Lady : "Orly? I have you on tape."
Guy : "Damn it, Fine."