Myth, originally a word of elevated and scholarly pretension, has passed into the vernacular to describe anything of dubious truth or validity, though it retains earlier senses. This post lists definitions of the word and others of which it is the root.
The word derives from the Greek term mythos, which variously means “speech” or “story,” or even “thought.” In modern English usage, a myth is a story, often featuring heroes and deities or supernatural entities, that explains a belief, custom, phenomenon, or worldview; it is also a synonym for allegory or parable. By extension, a myth is a belief or tradition, often one integral to a society, or an invalid notion born of ignorance or bigotry, or simply a rumor or untrue story. (Myth, without the article, denotes a body of myths.) An urban myth, meanwhile, is an account of an unusual or inexplicable event that did not occur, or state that does not exist, that is widely assumed to be true. The primary adjectival form is mythical; it is also used in the sense of “imaginary,” but mythic is appropriate for referring to astonishing achievements.
Mythology pertains to a set of myths, the study of myths, or an allegory, or to an assumption or belief. A mythologist (or, sometimes, mythologer) is someone who studies myths. Mythos is a synonym for both myth and mythology and denotes a symbolic set of cultural attitudes as well.
Mythogenesis is the development of myths or the tendency to ascribe mythical status to something. Mythopoeia, too, refers to the creation of myth. To mythicize is to turn something into, or treat something as, a myth; mythologize also has the latter meaning. Conversely, to demythologize is to analyze the meaning of myths or to unromanticize them. (A countermyth, meanwhile, is a myth that challenges or contradicts another myth.)
A mythmaker is someone who creates myths, generally in the casual sense of beliefs or traditions or of reputations about a person, place, or thing. A mythomaniac (or, sometimes, mythomane), meanwhile, is a pathological liar or exaggerator; the condition is called mythomania.
An etymologically related word is stichomythia (“verse speech”), denoting argumentative repartee, especially as a dramatic device in the performing arts.