In modern times, legendary is normally used as an adjective instead of a noun. The legendarium form is still found in several European languages, and was in occasional use in the English language when J. R. R. Tolkien used it to refer to some of his fictional writings about Middle-earth.
Tolkien's use of the term legendarium
Tolkien used the term legendarium with reference to his works in a total of four letters he wrote between 1951 and 1955, a period in which he was attempting to have his unfinished Silmarillion published alongside the more complete The Lord of the Rings:
On The Silmarillion: "This legendarium ends with a vision of the end of the world, its breaking and remaking, and the recovery of the Silmarilli and the 'light before the Sun' ...." (Letter to Milton Waldman, written c.1951)
On both texts "... my legendarium, especially the 'Downfall of Númenor' which lies immediately behind The Lord of the Rings, is based on my view: that Men are essentially mortal and must not try to become 'immortal' in the flesh." (Letter written in 1954)
On The Silmarillion: "Actually in the imagination of this story we are now living on a physically round Earth. But the whole 'legendarium' contains a transition from a flat world ... to a globe ...." (Letter written in 1954)
Encompassing both texts: "But the beginning of the legendarium, of which the Trilogy is part (the conclusion), was an attempt to reorganise some of the Kalevala ...." (Letter written in 1955)
Use of the phrase Tolkien's legendarium
"Tolkien's legendarium" is defined in the analytical work The History of the Hobbit by John D. Rateliff, as the body of Tolkien's work consisting of:
While other Tolkien scholars have not seen fit to define their use of the term, it is used in the following contexts:
Christopher Tolkien's introduction to The History of Middle-earth series, where he talks about the "primary 'legendarium'" in referring to core episodes and themes of The Silmarillion which were not abandoned in J.R.R. Tolkien's constant redrafting of the work.
Tolkien's Legendarium, a collection of critical essays on The History of Middle-earth edited by Verlyn Flieger and Carl F. Hostetter.
The following definition of The History of Middle-earth series in the J. R. R. Tolkien Encyclopedia: "The History of Middle-earth is a longitudinal study of the development and elaboration of Tolkien's legendarium through his transcribed manuscripts, with textual commentary by the editor, Christopher Tolkien."
Verlyn Flieger states "...the greatest is the creation of the Silmarils, the Gems of light that give their names to the whole legendarium", equating the legendarium concept with the Silmarillion (which itself is used to denote sometimes the work published under that name and sometimes the larger body of un-edited drafts used to create that work).
Dickerson and Evans use the phrase "legendarium" to encompass the entirety of Tolkien's Middle-earth writings 'for convenience'.