You can find definitions that suit your needs if you like, but it does not change the meaning of the word. A mythology is not necessarily untrue, although they

You can find definitions that suit your needs if you like, but it does not change the meaning of the word. A mythology is not necessarily untrue, although they do have a tendency toward expansiveness, which has a coincident trend toward favored interpretations. But the ultimate truthiness (if it can be measured at all) is neither requirement nor disqualification, vis a vis being a mythology.