blackmail

     Blackmail is an informal term for extortion, presumably extortion by mail or letter. Extortion is a crime for which the offender can be prosecuted--if caught. The extortionist finds out something about someone, and threatens to tell unless the victim pays. Sometimes this might be knowledge of a crime the victim has committed, but the more usual case would be attempting to entrap a prominent person through evidence of an illicit sexual liaison. For instance, a disreputable-looking woman might come up to someone in a public place and throw her arms around him, while someone snaps a picture. The picture is then "sold" to the victim of the extortionists. When extortionists use the mail--for example, by sending a copy of the photo described above, along with a demand for payment--it may still be called "blackmail". You can watch this in action in the Hollywood classic Elmer Gantry.