The Collaborative International Dictionary of English v.0.48:

Debenture stock \Debenture stock\ (Finance)
   The debt or series of debts, collectively, represented by a
   series of debentures; a debt secured by a trust deed of
   property for the benefit of the holders of shares in the debt
   or of a series of debentures. By the terms of much debenture
   stock the holders are not entitled to demand payment until
   the winding up of the company or default in payment; in the
   case of railway debentures, they cannot demand payment of the
   principal, and the debtor company cannot redeem the stock,
   except by authority of an act of Parliament. [Eng.]
   [Webster 1913 Suppl.]