Source : Webster's Revised Unabridged Dictionary (1913)

Debenture \De*ben"ture\ (?; 135), n. [L. debentur they are due,
   fr. debere to owe; cf. F. debentur. So called because these
   receipts began with the words Debentur mihi.]
   1. A writing acknowledging a debt; a writing or certificate
      signed by a public officer, as evidence of a debt due to
      some person; the sum thus due.

   2. A customhouse certificate entitling an exporter of
      imported goods to a drawback of duties paid on their
      importation. --Burrill.

   Note: It is applied in England to deeds of mortgage given by
         railway companies for borrowed money; also to municipal
         and other bonds and securities for money loaned.

Debenture \De*ben"ture\, n.
   Any of various instruments issued, esp. by corporations, as
   evidences of debt. Such instruments (often called

   {debenture bonds}) are generally, through not necessarily,
      under seal, and are usually secured by a mortgage or other
      charge upon property; they may be registered or
      unregistered. A debenture secured by a mortgage on
      specific property is called a

   {mortgage debenture}; one secured by a floating charge (which
      see), a

   {floating debenture}; one not secured by any charge

   {a naked debenture}. In general the term debenture in British
      usage designates any security issued by companies other
      than their shares, including, therefore, what are in the
      United States commonly called {bonds}. When used in the
      United States debenture generally designates an instrument
      secured by a floating charge junior to other charges
      secured by fixed mortgages, or, specif., one of a series
      of securities secured by a group of securities held in
      trust for the benefit of the debenture holders.

Source : WordNet®

debenture
     n 1: a bond that is backed by the credit of the issuer but not by
          any specific collateral [syn: {unsecured bond}, {debenture
          bond}] [ant: {secured bond}]
     2: a certificate or voucher acknowledging a debt