Real Dictionary

greenmail
uncountable noun: Greenmail is when a company buys enough shares in another company to threaten a takeover and makes a profit if the other company buys back its shares at a higher price.

greenmail in American English
the buying of a large amount of a company's stock in anticipation that the management, fearing that the buyer will gain control, will buy it back at a premium over the market price
noun: the practice of buying a large block of a company's stock in order to force a rise in stock prices or an offer by the company to repurchase that block of stock at an inflated price to thwart a possible takeover bid

greenmail in British English
noun: (esp in the US) the practice of a company buying sufficient shares in another company to threaten takeover and making a quick profit as a result of the threatened company buying back its shares at a higher price