The Collaborative International Dictionary of English v.0.48:

kickback \kick"back`\, n.
   1. recoil, of a gun or machine, as in older automobile
      engines when started by turning a crank.
      [PJC]

   2. A secret, and usually illegal, payment, by a recipient of
      money paid for goods or services, to a facilitator of the
      transaction, of a portion of that money; specifically, any
      portion of a gain made by the seller in a commercial
      transaction which is returned surreptitiously and
      illegally to a person (such as a purchasing agent) who
      authorized or recommended the purchase. It is generally
      considered a form of bribe.
      [PJC]

WordNet (r) 3.0 (2006):

kickback
    n 1: a commercial bribe paid by a seller to a purchasing agent
         in order to induce the agent to enter into the transaction