A stock of a company that is rumored to be target of a takeover attempt. Because of these rumors, company stocks are subject to speculations and high volatility, so trading them can bring large profits. Usually, when a takeover attempt is proclaimed, stocks will achieve greater price. Buying them before takeover proclamation will bring you great profit potential. On the other hand, a whisper is just a rumor; there are no guarantees that the takeover will actually take place.
However, trading on insider information basis is considered illegal. This is common term for companies that make a friendly takeover of a target company, while a target company already has a hostile takeover threat by Black Knight.