The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A customhouse certificate entitling an exporter of
      imported goods to a drawback of duties paid on their
      importation. --Burrill.
      [1913 Webster]

   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.
         [1913 Webster]

   3. 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.
      [Webster 1913 Suppl.]