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]