The Collaborative International Dictionary of English v.0.48:

Letter \Let"ter\, n. [OE. lettre, F. lettre, OF. letre, fr. L.
   littera, litera, a letter; pl., an epistle, a writing,
   literature, fr. linere, litum, to besmear, to spread or rub
   over; because one of the earliest modes of writing was by
   graving the characters upon tablets smeared over or covered
   with wax. --Pliny, xiii. 11. See Liniment, and cf.
   Literal.]
   1. A mark or character used as the representative of a sound,
      or of an articulation of the human organs of speech; a
      first element of written language.
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            And a superscription also was written over him in
            letters of Greek, and Latin, and Hebrew. --Luke
                                                  xxiii. 38.
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   2. A written or printed communication; a message expressed in
      intelligible characters on something adapted to
      conveyance, as paper, parchment, etc.; an epistle.
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            The style of letters ought to be free, easy, and
            natural.                              --Walsh.
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   3. A writing; an inscription. [Obs.]
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            None could expound what this letter meant.
                                                  --Chaucer.
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   4. Verbal expression; literal statement or meaning; exact
      signification or requirement.
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            We must observe the letter of the law, without doing
            violence to the reason of the law and the intention
            of the lawgiver.                      --Jer. Taylor.
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            I broke the letter of it to keep the sense.
                                                  --Tennyson.
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   5. (Print.) A single type; type, collectively; a style of
      type.
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            Under these buildings . . . was the king's printing
            house, and that famous letter so much esteemed.
                                                  --Evelyn.
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   6. pl. Learning; erudition; as, a man of letters.
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   7. pl. A letter; an epistle. [Obs.] --Chaucer.
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   8. (Teleg.) A telegram longer than an ordinary message sent
      at rates lower than the standard message rate in
      consideration of its being sent and delivered subject to
      priority in service of regular messages. Such telegrams
      are called by the Western Union Company day letters, or
      night letters according to the time of sending, and by
      The Postal Telegraph Company day lettergrams, or night
      lettergrams.
      [Webster 1913 Suppl.]

   Dead letter, Drop letter, etc. See under Dead, Drop,
      etc.

   Letter book, a book in which copies of letters are kept.

   Letter box, a box for the reception of letters to be mailed
      or delivered.

   Letter carrier, a person who carries letters; a postman;
      specif., an officer of the post office who carries letters
      to the persons to whom they are addressed, and collects
      letters to be mailed.

   Letter cutter, one who engraves letters or letter punches.


   Letter lock, a lock that can not be opened when fastened,
      unless certain movable lettered rings or disks forming a
      part of it are in such a position (indicated by a
      particular combination of the letters) as to permit the
      bolt to be withdrawn.
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            A strange lock that opens with AMEN.  --Beau. & Fl.

   Letter paper, paper for writing letters on; especially, a
      size of paper intermediate between note paper and
      foolscap. See Paper.

   Letter punch, a steel punch with a letter engraved on the
      end, used in making the matrices for type.

   Letters of administration (Law), the instrument by which an
      administrator or administratrix is authorized to
      administer the goods and estate of a deceased person.

   Letter of attorney, Letter of credit, etc. See under
      Attorney, Credit, etc.

   Letter of license, a paper by which creditors extend a
      debtor's time for paying his debts.

   Letters close or Letters clause (Eng. Law.), letters or
      writs directed to particular persons for particular
      purposes, and hence closed or sealed on the outside; --
      distinguished from letters patent. --Burrill.

   Letters of orders (Eccl.), a document duly signed and
      sealed, by which a bishop makes it known that he has
      regularly ordained a certain person as priest, deacon,
      etc.

   Letters patent, Letters overt, or Letters open (Eng.
      Law), a writing executed and sealed, by which power and
      authority are granted to a person to do some act, or enjoy
      some right; as, letters patent under the seal of England.
      The common commercial patent is a derivative form of
      such a right.

   Letter-sheet envelope, a stamped sheet of letter paper
      issued by the government, prepared to be folded and sealed
      for transmission by mail without an envelope.

   Letters testamentary (Law), an instrument granted by the
      proper officer to an executor after probate of a will,
      authorizing him to act as executor.

   Letter writer.
      (a) One who writes letters.
      (b) A machine for copying letters.
      (c) A book giving directions and forms for the writing of
          letters.
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The Collaborative International Dictionary of English v.0.48:

Credit \Cred"it\ (kr[e^]d"[i^]t), n. [F. cr['e]dit (cf. It.
   credito), L. creditum loan, prop. neut. of creditus, p. p. of
   credere to trust, loan, believe. See Creed.]
   1. Reliance on the truth of something said or done; belief;
      faith; trust; confidence.
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            When Jonathan and the people heard these words they
            gave no credit unto them, nor received them. --1
                                                  Macc. x. 46.
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   2. Reputation derived from the confidence of others; esteem;
      honor; good name; estimation.
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            John Gilpin was a citizen
            Of credit and renown.                 --Cowper.
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   3. A ground of, or title to, belief or confidence; authority
      derived from character or reputation.
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            The things which we properly believe, be only such
            as are received on the credit of divine testimony.
                                                  --Hooker.
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   4. That which tends to procure, or add to, reputation or
      esteem; an honor.
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            I published, because I was told I might please such
            as it was a credit to please.         --Pope.
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   5. Influence derived from the good opinion, confidence, or
      favor of others; interest.
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            Having credit enough with his master to provide for
            his own interest.                     --Clarendon.
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   6. (Com.) Trust given or received; expectation of future
      playment for property transferred, or of fulfillment or
      promises given; mercantile reputation entitling one to be
      trusted; -- applied to individuals, corporations,
      communities, or nations; as, to buy goods on credit.
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            Credit is nothing but the expectation of money,
            within some limited time.             --Locke.
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   7. The time given for payment for lands or goods sold on
      trust; as, a long credit or a short credit.
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   8. (Bookkeeping) The side of an account on which are entered
      all items reckoned as values received from the party or
      the category named at the head of the account; also, any
      one, or the sum, of these items; -- the opposite of
      debit; as, this sum is carried to one's credit, and that
      to his debit; A has several credits on the books of B.
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   Bank credit, or Cash credit. See under Cash.

   Bill of credit. See under Bill.

   Letter of credit, a letter or notification addressed by a
      banker to his correspondent, informing him that the person
      named therein is entitled to draw a certain sum of money;
      when addressed to several different correspondents, or
      when the money can be drawn in fractional sums in several
      different places, it is called a circular letter of
      credit.

   Public credit.
      (a) The reputation of, or general confidence in, the
          ability or readiness of a government to fulfill its
          pecuniary engagements.
      (b) The ability and fidelity of merchants or others who
          owe largely in a community.
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                He touched the dead corpse of Public Credit, and
                it sprung upon its feet.          --D. Webster.
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WordNet (r) 3.0 (2006):

letter of credit
    n 1: a document issued by a bank that guarantees the payment of
         a customer's draft; substitutes the bank's credit for the
         customer's credit

Moby Thesaurus II by Grady Ward, 1.0:

75 Moby Thesaurus words for "letter of credit":
   CD, IOU, MO, Pastoral Epistle, acceptance, acceptance bill,
   aerogram, air letter, airgraph, bank acceptance, bank check, bill,
   bill of draft, bill of exchange, billet-doux, blank check, bull,
   certificate, certificate of deposit, certified check, chain letter,
   charge card, charge plate, check, checkbook, cheque, circular note,
   commercial paper, credit card, credit instrument, credit slip,
   dead letter, debenture, demand bill, demand draft, deposit slip,
   dimissorial, dimissory letter, draft, drop letter, due bill,
   encyclical, exchequer bill, fan letter, form letter,
   letter of introduction, letters credential, letters of marque,
   letters of request, letters overt, letters patent,
   letters rogatory, love letter, money order, monitory,
   negotiable instrument, newsletter, nixie, note, note of hand,
   open letter, paper, pastoral letter, poison-pen letter,
   postal order, promissory note, round robin, sight bill,
   sight draft, time bill, time draft, trade acceptance,
   treasury bill, voucher, warrant



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

LETTER OF CREDIT, contracts. An open or sealed letter, from a merchant in
one place, directed to another, in another place or country, requiring him
that if a person therein named, or the bearer of the letter, shall have
occasion to buy commodities, or to want money to any particular or unlimited
amount, either to procure the same, or to pass his promise, bill, or other
engagement for it, the writer of the letter undertaking to provide him the
money for the goods, or to repay him by exchange, or to give him such
satisfaction as he shall require, either for himself or the bearer of the
letter. 3 Chit Com. Law, 336; and see 4 Chit. Com. Law, 259, for a form of
such letter.
     2. These letters are either general or special; the former is directed
to the writer's friends or correspondents generally, where the bearer of the
letter may happen to go; the latter is directed to some particular person.
When the letter is presented to the person to whom it is addressed, he
either agrees to comply with the request, in which case he immediately
becomes bound to fulfill all the engagements therein mentioned; or he
refuses in which case the bearer should return it to the giver without any
other proceeding, unless, indeed, the merchant to whom the letter is
directed is a debtor of the merchant who gave the letter, in which case he
should procure the letter to be protested. 3 Chit. Com. Law, 337; Mal., 76;
1 Beawes. 607; Hall's Adm. Pr. 14; 4 Ohio R. 197; 1 Wilc. R. 510.
     3. The debt which arises on such letter, in its simplest form, when
complied with, is between the mandator and the mandant; though it may be so
conceived as to raise a debt also against the person who is supplied by the
mandatory. 1. When the letter is purchased with money by the person wishing
for the foreign credit; or, is granted in consequence of a check on his cash
account, or procured on the credit of securities lodged with the person who
granted it; or in payment of money due by him to the payee; the letter is,
in its effects, similar to a bill of exchange drawn on the foreign merchant.
The payment of the money by the person on whom the letter is granted raises
a debt, or goes into account between him and the writer of the letter; but
raises no debt to the person who pays on the letter, against him to whom the
money is paid. 2. When not so purchased, but truly an accommodation, and
meant to raise a debt on the person accommodated, the engagement, generally
is, to see paid any advances made to him, or to guaranty any draft accepted
or bill discounted and the compliance with the mandate, in such case, raises
a debt, both against the writer of the letter, and against the person
accredited. 1 Bell's Com. 371, 6th ed. The bearer of the letter of credit is
not considered bound to receive the money; he may use the letter as he
pleases, and he contracts an obligation only by receiving the money. Poth.
Contr. de Change, 237.