I define as myth
the claim to reality of a verbal account 
a) of events or circumstances in times past, or 
b) of events or circumstances predicted to occur or recur 
at some juncture in the future, 
the occurrence or recurrence of which, 
for whatever reason, has not (yet) supervened.
With the occurrence of the hypothesized event, the myth dissipates 
and the potential which the myth anticipated is realized,
but  dissipates (perhaps) only transiently, because having occurred,
it slips (merges) into history, and becomes historical myth.
c) of events which by their nature can only be inferred,
but which events can never be directly experienced,
e.g., the preponderance of physics theory,
e.g., the physics of Newton, Maxwell, Einstein, Planck.

The reality which resists being or becoming myth, is the reality which is:
a) constant, e.g. gravity, or
b) recurring at regular, predictable intervals,
e.g, the seasons, the diurnal cycle, the pulse of the heart beat,
the respiratory cycle, human life itself, although the individual's human life,
his "autobiography", unavoidably fades into myth with the passage of time.