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