The Collaborative International Dictionary of English v.0.48:

Voucher \Vouch"er\, n.
   1. One who vouches, or gives witness or full attestation, to
      anything.
      [1913 Webster]

            Will his vouchers vouch him no more?  --Shak.
      [1913 Webster]

            The great writers of that age stand up together as
            vouchers for one another's reputation. --Spectator.
      [1913 Webster]

   2. A book, paper, or document which serves to vouch the truth
      of accounts, or to confirm and establish facts of any
      kind; also, any acquittance or receipt showing the payment
      of a debt; as, the merchant's books are his vouchers for
      the correctness of his accounts; notes, bonds, receipts,
      and other writings, are used as vouchers in proving facts.
      [1913 Webster]

   3. (Law)
      (a) The act of calling in a person to make good his
          warranty of title in the old form of action for the
          recovery of lands.
      (b) The tenant in a writ of right; one who calls in
          another to establish his warranty of title. In common
          recoveries, there may be a single voucher or double
          vouchers. --Blackstone.
          [1913 Webster]

   4. A document attesting to a credit against certain defined
      expenditures; a recipt for prepayment; -- often used in
      pre-arranged travel plans, to provide evidence of
      pre-payment of the cost of lodging, transportation, or
      meals.
      [PJC]

WordNet (r) 3.0 (2006):

voucher
    n 1: someone who vouches for another or for the correctness of a
         statement [syn: voucher, verifier]
    2: a document that serves as evidence of some expenditure
    3: a negotiable certificate that can be detached and redeemed as
       needed [syn: coupon, voucher]

Moby Thesaurus II by Grady Ward, 1.0:

94 Moby Thesaurus words for "voucher":
   CD, IOU, MO, acceptance, acceptance bill, acknowledgment,
   acquittance, advocacy, advocating, advocation, affidavit,
   attestant, attestation, attestator, attester, authority,
   authorization, bank acceptance, bank check, bill, bill of draft,
   bill of exchange, bill of health, blank check, bystander,
   canceled check, certificate, certificate of character,
   certificate of deposit, certificate of proficiency, certification,
   certified check, character, character reference, check, checkbook,
   cheque, cojuror, commercial paper, compurgator, credential,
   debenture, demand bill, demand draft, deponent, deposition,
   diploma, discharge, draft, due bill, earwitness, exchequer bill,
   eyewitness, informant, informer, letter of credit,
   letter of introduction, money order, navicert,
   negotiable instrument, notarized statement, note, note of hand,
   paper, passerby, patronage, postal order, promissory note,
   quittance, receipt, receipt in full, recommend, recommendation,
   reference, release, sheepskin, sight bill, sight draft, spectator,
   swearer, sworn statement, testamur, testifier, testimonial, ticket,
   time bill, time draft, trade acceptance, treasury bill, visa, vise,
   warrant, warranty, witness



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

VOUCHER, accounts. An account book in which are entered the acquittances, or
warrants for the accountant's discharge. It also signifies any acquittance
or receipt, which is evidence of payment, or of the debtor's being
discharged. See 3 Halst. 299.



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

VOUCHER, common recoveries. The voucher in common recoveries, is the person
on whom the tenant to the praecipe calls to defend the title to the land,
because he is supposed to have warranted the title to him at the time of the
original purchase.
     2. The person usually employed for this purpose is the cryer of the
court, who is therefore called the common voucher. Vide Cruise, Dig. tit.
36, c. 3, s. 1; 22 Vin. Ab. 26; Dane, Index, h.t.; and see Recovery.