Means it's a common misperception, or a downright lie. These are all valid definitions of myth in popular English usage. We can toss a couple more definitions into the pot.
Myth is the other guy's religion, which is, of course, a misguided belief.
Myths are stories about gods and goddesses. From Herodotus on, the Greeks found it convenient to distinguish between fictional story, for which they used the word mythos , and matters of fact, which for which they used the word logos . These megalithic statues are moai on Easter Island in the South Pacific about 2300 miles off the coast of Chile. The moai were carved from the island's volcanic rock to draw down the vital power of the gods. Perhaps we can arrive at a fairly workable idea of what myth is by examining what it does.