The Collaborative International Dictionary of English v.0.48:

Hush \Hush\, n.
   Stillness; silence; quiet. [R.] "It is the hush of night."
   --Byron.
   [1913 Webster]

   Hush money, money paid to secure silence, or to prevent the
      disclosure of facts. --Swift.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hush money
    n 1: a bribe paid to someone to insure that something is kept
         secret

Moby Thesaurus II by Grady Ward, 1.0:

26 Moby Thesaurus words for "hush money":
   account, allowance, assessment, bill, blackmail, blood money,
   boodle, bribe, bribe money, emolument, fee, footing, gratification,
   gratuity, initiation fee, mileage, payoff, payola, protection,
   reckoning, retainer, retaining fee, scot, sop, stipend, tribute