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.]

WordNet (r) 3.0 (2006):

debenture
    n 1: the ability of a customer to obtain goods or services
         before payment, based on the trust that payment will be
         made in the future [syn: unsecured bond, debenture,
         debenture bond] [ant: secured bond]
    2: a certificate or voucher acknowledging a debt

Moby Thesaurus II by Grady Ward, 1.0:

142 Moby Thesaurus words for "debenture":
   CD, Fannie Mae, Federal Agency bond, Ginnie Mae, IOU, MO,
   Series E bond, Series H bond, acceptance, acceptance bill,
   adjustment bond, annuity bond, appreciation bond, arrangement,
   assented bond, assumed bond, baby bond, bank acceptance,
   bank check, bearer bond, bearer certificate, bill, bill of draft,
   bill of exchange, blank check, bond, bond anticipation note,
   callable bond, certificate, certificate of deposit,
   certified check, check, checkbook, cheque, collateral trust bond,
   commercial paper, consolidated annuities, consolidated stock,
   consols, contract by deed, contract of record, contract quasi,
   convertible bond, convertible debenture, corporate bond,
   corporation stock, coupon bond, coupon rate, covenant of indemnity,
   current income bond, current yield, debenture bond, deed,
   deed of trust, deed poll, deep-discount bond, defense bond,
   deferred bond, definitive bond, demand bill, demand draft,
   discount bond, draft, due bill, equipment bond, equipment note,
   equipment trust, equipment trust bond, equipment trust certificate,
   exchequer bill, extended bond, first mortgage bond, firsts,
   formal contract, general mortgage bond, general obligation bond,
   government bond, group policy, guaranteed bond, high-grade bond,
   implied contract, income bond, indent, indenture, insurance policy,
   interchangeable bond, interim bond, joint bond, letter of credit,
   money order, mortgage bond, mortgage deed, municipal bond,
   negotiable bond, negotiable instrument, nominal rate,
   nonnegotiable bond, note, note of hand, optional bond, paper,
   par bond, parol contract, participating bond, perpetual bond,
   policy, postal order, premium bond, promissory note,
   purchase money bond, recognizance, refunding bond, registered bond,
   registered certificate, revenue bond, savings bond,
   second mortgage bond, seconds, secured bond, sight bill,
   sight draft, small bond, special contract, specialty,
   specialty contract, state bond, tax anticipation note,
   tax-free bond, time bill, time draft, title deed, trade acceptance,
   treasury bill, treasury bond, trust indenture,
   trustee mortgage bond, unsecured bond, voting bond, voucher,
   war bond, warrant, yield to maturity



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DEBENTURE. A certificate given, in pursuance of law, by the collector of a
port of entry, for a certain sum, due by the United States, payable at a
time therein mentioned, to an importer for drawback of duties on merchandise
imported and exported by him, provided the duties arising on the importation
of the said merchandise shall have been discharged prior to the time
aforesaid. Vide Act of Congress of March 2, 1799, s. 80; Encyclopedie, h.t.;
Dane's Ab. Index, h.t.