The Collaborative International Dictionary of English v.0.48:

Myth \Myth\ (m[i^]th), n. [Written also mythe.] [Gr. my^qos
   myth, fable, tale, talk, speech: cf. F. mythe.]
   1. A story of great but unknown age which originally embodied
      a belief regarding some fact or phenomenon of experience,
      and in which often the forces of nature and of the soul
      are personified; an ancient legend of a god, a hero, the
      origin of a race, etc.; a wonder story of prehistoric
      origin; a popular fable which is, or has been, received as
      historical.
      [1913 Webster]

   2. A person or thing existing only in imagination, or whose
      actual existence is not verifiable.
      [1913 Webster]

            As for Mrs. Primmins's bones, they had been myths
            these twenty years.                   --Ld. Lytton.
      [1913 Webster]

   Myth history, history made of, or mixed with, myths.
      [1913 Webster]

WordNet (r) 3.0 (2006):

myth
    n 1: a traditional story accepted as history; serves to explain
         the world view of a people

Moby Thesaurus II by Grady Ward, 1.0:

100 Moby Thesaurus words for "myth":
   Marchen, Mishnah, Spiritus Mundi, Sunna, Talmud, Western,
   Western story, Westerner, adventure story, allegory,
   ancient wisdom, apologue, apparition, archetypal myth,
   archetypal pattern, bedtime story, brainchild, bubble, canard,
   chimera, cock-and-bull story, common law, concoction, creation,
   custom, delirium, detective story, eidolon, epic, extravaganza,
   fable, fabliau, fabrication, fairy tale, falsehood, fancy,
   fantasque, fantasy, fib, fiction, figment, folk motif, folk story,
   folklore, folktale, forgery, gest, ghost story, hallucination,
   history, horse opera, idle fancy, illusion, imagery, imagination,
   imagining, immemorial usage, insubstantial image, invention,
   legend, lie, lore, love story, maggot, make-believe, mystery,
   mystery story, mythology, mythos, nursery tale, parable, phantasm,
   phantom, prevarication, racial memory, romance, saga,
   science fiction, shocker, sick fancy, space fiction, space opera,
   story, suspense story, tall tale, thick-coming fancies, thriller,
   tradition, traditionalism, traditionality, trip, untruth, vapor,
   vision, whim, whimsy, whodunit, whopper, wildest dreams,
   work of fiction