"Greenmail" is the practice of acquiring a large portion of the shares of a company and then forcing the issuing company to buy enough of the shares back to avoid a hostile takeover. It is similar to blackmail and can be effective because a company wishing to thwart the takeover may have to pay the holder of the shares an inflated price to get buy the shares back. Some corporate charters include anti-greenmail provisions that restrict a company's ability to buy back shares in this situation.