The Collaborative International Dictionary of English v.0.48:

Writer \Writ"er\, n. [AS. wr[imac]tere.]
   [1913 Webster]
   1. One who writes, or has written; a scribe; a clerk.
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            They [came] that handle the pen of the writer.
                                                  --Judg. v. 14.
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            My tongue is the pen of a ready writer. --Ps. xlv.
                                                  1.
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   2. One who is engaged in literary composition as a
      profession; an author; as, a writer of novels.
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            This pitch, as ancient writers do report, doth
            defile.                               --Shak.
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   3. A clerk of a certain rank in the service of the late East
      India Company, who, after serving a certain number of
      years, became a factor.
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   Writer of the tallies (Eng. Law), an officer of the
      exchequer of England, who acted as clerk to the auditor of
      the receipt, and wrote the accounts upon the tallies from
      the tellers' bills. The use of tallies in the exchequer
      has been abolished. --Wharton (Law. Dict.)

   Writer's cramp, Writer's palsy or Writer's spasm
      (Med.), a painful spasmodic affection of the muscles of
      the fingers, brought on by excessive use, as in writing,
      violin playing, telegraphing, etc. Called also
      scrivener's palsy.

   Writer to the signet. See under Signet.
      [1913 Webster]

WordNet (r) 3.0 (2006):

writer
    n 1: writes (books or stories or articles or the like)
         professionally (for pay) [syn: writer, author]
    2: a person who is able to write and has written something

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "writer":
   addressee, advertising writer, amanuensis, annalist, art critic,
   author, authoress, belletrist, bibliographer, calligrapher,
   chirographer, clerk, coauthor, collaborator, columnist,
   communicator, compiler, composer, copier, copyist, copywriter,
   correspondent, creative writer, critic, dance critic, descanter,
   diarist, discourser, disquisitor, drama critic, dramatist,
   encyclopedist, essayist, expositor, free lance, free-lance writer,
   ghost, ghostwriter, humorist, inditer, ink spiller, inkslinger,
   letter writer, letterer, literary artist, literary craftsman,
   literary critic, literary man, litterateur, logographer,
   magazine writer, man of letters, monographer, monographist,
   music critic, newspaperman, novelettist, novelist, pamphleteer,
   pen, pen pal, pencil driver, penman, penner, penwoman, poet,
   prose writer, recording secretary, reviewer, scenario writer,
   scenarist, scribbler, scribe, scriptwriter, scrivener, secretary,
   short-story writer, storyteller, technical writer, tractation,
   transcriber, word painter, word-slinger, wordsmith