From Citizendium, the Citizens' Compendium
The word myth has two main meanings related to a story that is widely believed but false. In order to be mythic, as opposed to fiction, the story must have some longevity, and often some grandiose concept or proportion. All myths are fiction, but not all fiction is mythic. Stories become myths over time.
In general use, the concept of the myth, which can be pejorative, arose from the recognition that many myths from antiquity were not historically factual, therefore false knowledge. This usage is frequently confused with the related terms legend, fairy tale, folklore, fable, and urban legend, each of which has a distinct meaning.
Academic use of the term 'myth'
In the academic fields of Mythology, mythography, and folkloristics, a myth (mythos) is a sacred story concerning the origins of the world or how the world and the creatures in it came to have their present form. The active beings in myths are generally gods and heroes. Myths often are said to take place before recorded history begins. In saying that a myth is a sacred narrative, what is meant is that a myth is believed to be true by people who attach religious or spiritual significance to it. In this context, use of the term by scholars does not imply that the narrative is either true or false.
A secular myth is a story which powerfully illustrates an aspect of the originating culture; its history, values, aspirations, or heroics. For example, the culture of the United States has developed myths of the 'superhero,' most notably: Superman.