For questions about mythology, stories or beliefs about supernatural beings and events held to be true in some culture.
A myth is a story or belief about supernatural beings and events held to be true in some culture. A body of myths, or the study of myths, is called mythology.
Mythology is distinct from folklore in that mythology was or is held to be accurate and true, and often sacred. For example, Odin is a figure from mythology; Santa Claus is a figure from folklore. Sometimes figures move between the two categories as the culture changes - one may find elements from myths about Odin in folktales about Santa Claus.
If your question is about mythology itself, not how it is portrayed in a science fiction or fantasy fiction work, you should probably ask it on the Mythology Stack Exchange website instead of here.
In discussion of speculative fiction we sometimes operate one step removed and talk about the mythology of a particular work, for example, "the mythology of the-x-files." In these situations the word refers to the stories or beliefs held to be true within a culture within that work.