The Collaborative International Dictionary of English v.0.48:

Fiction \Fic"tion\, n. [F. fiction, L. fictio, fr. fingere,
   fictum to form, shape, invent, feign. See Feign.]
   1. The act of feigning, inventing, or imagining; as, by a
      mere fiction of the mind. --Bp. Stillingfleet.
      [1913 Webster]

   2. That which is feigned, invented, or imagined; especially,
      a feigned or invented story, whether oral or written.
      Hence: A story told in order to deceive; a fabrication; --
      opposed to fact, or reality.
      [1913 Webster]

            The fiction of those golden apples kept by a dragon.
                                                  --Sir W.
                                                  Raleigh.
      [1913 Webster]

            When it could no longer be denied that her flight
            had been voluntary, numerous fictions were invented
            to account for it.                    --Macaulay.
      [1913 Webster]

   3. Fictitious literature; comprehensively, all works of
      imagination; specifically, novels and romances.
      [1913 Webster]

            The office of fiction as a vehicle of instruction
            and moral elevation has been recognized by most if
            not all great educators.              --Dict. of
                                                  Education.
      [1913 Webster]

   4. (Law) An assumption of a possible thing as a fact,
      irrespective of the question of its truth. --Wharton.
      [1913 Webster]

   5. Any like assumption made for convenience, as for passing
      more rapidly over what is not disputed, and arriving at
      points really at issue.

   Syn: Fabrication; invention; fable; falsehood.

   Usage: Fiction, Fabrication. Fiction is opposed to what
          is real; fabrication to what is true. Fiction is
          designed commonly to amuse, and sometimes to instruct;
          a fabrication is always intended to mislead and
          deceive. In the novels of Sir Walter Scott we have
          fiction of the highest order. The poems of Ossian, so
          called, were chiefly fabrications by Macpherson.
          [1913 Webster]

WordNet (r) 3.0 (2006):

fiction
    n 1: a literary work based on the imagination and not
         necessarily on fact
    2: a deliberately false or improbable account [syn:
       fabrication, fiction, fable]

Moby Thesaurus II by Grady Ward, 1.0:

153 Moby Thesaurus words for "fiction":
   Marchen, Western, Western story, Westerner, adventure story,
   allegory, anecdote, apologue, apparition, article, autograph,
   bedtime story, blague, brainchild, bubble, canard, chimera,
   cock-and-bull story, composition, computer printout, concoction,
   copy, delirium, detective story, document, draft, edited version,
   eidolon, engrossment, essay, exaggeration, extravaganza, fable,
   fabliau, fabrication, fair copy, fairy tale, falsehood, falsity,
   fancy, fantasque, fantasy, farfetched story, farrago, fib, figment,
   final draft, finished version, first draft, fish story, flam,
   flimflam, flimsy, folk story, folktale, forgery, gest, ghost story,
   half-truth, hallucination, holograph, horse opera, idle fancy,
   illusion, imagery, imagination, imagining, insubstantial image,
   invention, legal fiction, legend, letter, lie, literae scriptae,
   literary artefact, literary production, literature,
   little white lie, love story, lucubration, maggot, make-believe,
   manuscript, matter, mendacity, misrepresentation, mystery,
   mystery story, myth, mythology, mythos, narrative, nonfiction,
   nursery tale, opus, original, paper, parable, parchment, penscript,
   phantasm, phantom, piece, piece of writing, pious fiction, play,
   poem, prevarication, printed matter, printout, production,
   reading matter, recension, romance, science fiction, screed, scrip,
   script, scrive, scroll, second draft, shocker, sick fancy,
   slight stretching, space fiction, space opera, story,
   suspense story, tale, tall story, tall tale, taradiddle,
   the written word, thick-coming fancies, thriller, transcript,
   transcription, trip, trumped-up story, typescript, untruth, vapor,
   version, vision, whim, whimsy, white lie, whodunit, wildest dreams,
   work, work of fiction, writing, yarn