The Cthulhu Mythos

The Cthulhu Mythos is a shared fictional universe, based on the work of American horror writer H. P. Lovecraft.

The term was first coined by August Derleth, a contemporary correspondent of Lovecraft, who used the name of the creature Cthulhu-a central figure in Lovecraft literature and the focus of Lovecraft's short story "The Call of Cthulhu" (first published in pulp magazine Weird Tales in 1928)-to identify the system of lore employed by Lovecraft and his literary successors. The writer Richard L. Tierney later applied the term "Derleth Mythos" to distinguish between Lovecraft's works and Derleth's later stories.

Authors writing in the Lovecraftian milieu use elements of the Mythos in an ongoing expansion of the fictional universe
Robert M. Price described, in his essay "H. P. Lovecraft and the Cthulhu Mythos," two stages in the development of the Cthulhu Mythos. The first stage, termed the "Cthulhu Mythos proper" by Price, was formulated during Lovecraft's lifetime and was subject to his guidance. The second stage was guided by August Derleth who, in addition to publishing Lovecraft's stories after his death, attempted to categorize and expand the Mythos.

A lesser known term employed by the scholar S. T. Joshi to describe the works of Lovecraft. Joshi identified four key elements in Lovecraft's mythos (that Price would later condense to three themes), being the fundamental principle of cosmicism (which once again highlighted the irrelevance of mankind), the imaginary New England setting, a pantheon of recurring "pseudomythological" entities and a collection of arcane books that supposedly yield insights into the mythology.