Source : Webster's Revised Unabridged Dictionary (1913)

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.

            And a superscription also was written over him in
            letters of Greek, and Latin, and Hebrew. --Luke
                                                  xxiii. 38.

   2. A written or printed communication; a message expressed in
      intelligible characters on something adapted to
      conveyance, as paper, parchment, etc.; an epistle.

            The style of letters ought to be free, easy, and
            natural.                              --Walsh.

   3. A writing; an inscription. [Obs.]

            None could expound what this letter meant.
                                                  --Chaucer.

   4. Verbal expression; literal statement or meaning; exact
      signification or requirement.

            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.

            I broke the letter of it to keep the sense.
                                                  --Tennyson.

   5. (Print.) A single type; type, collectively; a style of
      type.

            Under these buildings . . . was the king's printing
            house, and that famous letter so much esteemed.
                                                  --Evelyn.

   6. pl. Learning; erudition; as, a man of letters.

   7. pl. A letter; an epistle. [Obs.] --Chaucer.

   {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.

            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 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}, {overt}, or {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.

   {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.



   {Dead letter}.
       (a) A letter which, after lying for a certain fixed time
           uncalled for at the post office to which it was
           directed, is then sent to the general post office to
           be opened.
       (b) That which has lost its force or authority; as, the
           law has become a dead letter.

   {Dead-letter office}, a department of the general post office
      where dead letters are examined and disposed of.

   {Dead level}, a term applied to a flat country.

   {Dead lift}, a direct lift, without assistance from
      mechanical advantage, as from levers, pulleys, etc.;
      hence, an extreme emergency. ``(As we say) at a dead
      lift.'' --Robynson (More's Utopia).

   {Dead line} (Mil.), a line drawn within or around a military
      prison, to cross which involves for a prisoner the penalty
      of being instantly shot.

   {Dead load} (Civil Engin.), a constant, motionless load, as
      the weight of a structure, in distinction from a moving
      load, as a train of cars, or a variable pressure, as of
      wind.

   {Dead march} (Mus.), a piece of solemn music intended to be
      played as an accompaniment to a funeral procession.

   {Dead nettle} (Bot.), a harmless plant with leaves like a
      nettle ({Lamium album}).

   {Dead oil} (Chem.), the heavy oil obtained in the
      distillation of coal tar, and containing phenol,
      naphthalus, etc.

Source : WordNet®

dead letter
     n 1: the state of something that has outlived its relevance [syn:
           {non-issue}]
     2: mail that can neither be delivered nor returned [syn: {dead
        mail}]