what is myth in philosophical terms?

A non-logical narrative story that seeks to explain accepted notions of reality for the benefits of the tribe, the regime, etc. Some myths can be used for the benefit of protecting philosophy (Plato's noble lie).

A myth is a story which explains or enlightens - the details of the story may not be objectively `true' but the Truth is revealed in the telling of the tale. Example: the details of the myth of Demeter and Persephone may not be `factual' but it is true that seasons change and that there is a reason why they do so. To the ancients, the details of a story were not so important as the message that story related. You would never find a group of people in, say, 180 CE, arguing about the differences in the narratives of the four gospels of the Christian Bible. They wouldn't have cared who saw Jesus first after the crucifixion and wouldn't have seen any problems with what modern-day readers view as contradictions. What mattered was the `truth' in the tale told, not the details of the telling. I'll put a site which you may find useful in the Sources section here. All the best to you.......

According to Alan Watts "A myth is an image in terms of which we try to make sense of the world."

It's a symbol we use to describe or try to understand a certain phenomena or aspect of reality.

A happening,without proof