Hush money is an arrangement in which one person or party offers another money or other enticement, in exchange for remaining silent about some illegal, stigmatized, or shameful behavior, action, or other fact about the person or party who has made the offer.
[1] It can also be money paid to placate a disgruntled adversary who may disclose embarrassing information, even if untrue, to avoid the harm and trouble of dealing with defamation.
The person or party who presents the hush money may be attempting to avoid criminal prosecution, a lawsuit (as sometimes in the case of an out-of-court settlement), a leak of information to the news media, or silence about a stigmatized issue, politicians and their appointees, or a national government in its standing among other nations in the world.
The latter type of agreement can be a criminal act itself as an obstruction of justice or perjury.
The Oxford English Dictionary traces published use of the term to Richard Steele in 1709.