J.R.R. Tolkien’s large collection of myths and histories behind Middle-earth is commonly called his legendarium.
The term describes the interconnected body of stories, languages, genealogies, cosmology, and historical texts that Tolkien developed over much of his life. It includes material later published in The Silmarillion, The Hobbit, The Lord of the Rings, and posthumous collections.
Tolkien did not publish a single definitive book titled The Legendarium. The word is a modern descriptive term used by readers and scholars for the evolving fictional mythology he created. The contents changed repeatedly as Tolkien revised names, events, languages, and chronology.
The legendarium is broader than Middle-earth alone. Arda is the name of the world, while Aman is a major continent within it. The Red Book of Westmarch is a fictional source text within the stories, not the name of Tolkien’s entire body of mythology.