Webster's Definition of Myth

Myth is "a story that is usually of unknown origin and at least partly traditional, that ostensibly relates historical events usually of such a character as to serve to explain some practice, belief, institution, or natural phenomenon, and that is especially associated with religious rites and beliefs."

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Morford and Lenardon's Definition (p. 25)

"A classical myth is a story that, through its classical form, has attained a kind of immortality because its inherent archetypal beauty, profundity, and power have inspired rewarding renewal and transformation by successive generations."