The Collaborative International Dictionary of English v.0.48:

greenmail \green"mail\ n. (Finance)
   The act, performed by a publicly traded corporation, of
   paying a corporate raider to give up a takeover attempt, by
   buying the shares of stock he owns; also, the threat posed by
   corporate raiders to take over a company unless their stocks
   are purchased by the company at a price giving them a large
   profit. [Informal]
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

greenmail
    n 1: (corporation) the practice of purchasing enough shares in a
         firm to threaten a takeover and thereby forcing the owners
         to buy those shares back at a premium in order to stay in
         business