Legend

A legend is a story which is passed on person-to-person, and which has important ontological consequences. It is a concept, an ideal or a semi-true story with mythic qualities, usually involving an heroic character or fantastic place, and rooted in a kernel of truth; it is a meme that propagates through a culture.

Some legends we know today have their basis in historical fact. Before the invention of the printing press, stories were passed on via oral tradition. Storytellers abounded. They learned their stock in trade, their stories, typically from an older storyteller, who might (or might not) have actually been there when the "story" was "history" bardic schools.

Examples:

Atlantis
King Arthur and the Knights of the Round Table
The Holy Grail
El Dorado
The Fountain of Youth
Robin Hood
Roland

A legend is different from a conspiracy and conspiracy theories in that the lynchpin of the latter two is usually a plausible, but unprovable secret agenda which exclusively drives the story.

Cosmology is usually myth.