This question was closed - by a unilateral moderator vote - as "General Reference".
When researching LOTR lore, I frequently ran into a term "legendarium" as pertaining to the entirety of Tolkien's work (followed by 3 specific subquestions).
As a background, before asking the question, I have confirmed that:
The term was NOT defined on:
Wikipedia's "Legendarium" redirects straight to Tolkien page.
Also, that page on top has a note stating ""Legendarium" redirects here. For other uses, see Legendary (disambiguation)" - and that disambiguation page shows:
Legendary may refer to:
- A hagiography, or study of the lives of saints and other religious figures The South English Legendary, a Middle English legendary
Note that the proper English term seems to be "Legendary" (a noun), and "Legendarium" is the Latin word it derived from (this will be important later).
Google's "define:legendarium" search first 3 pages were exclusively Tolkien hits, with the only exception being "Anjou Legendarium" - which was a Hungarian work written presumably in Latin? - and definitely with a Latin name - see above bullet.
Google search for "Legendarium" also returns almost exclusively Tolkien content with 2 hits to "Anjou Legendarium" sprinkled in.
When asked in a comment to explain the closure, the following arguments were made, which seem to me to be invalid:
"I'm going to refer you to a dictionary. Wiktionary defines the word as “a literary collection of legends, as of a saint”."
However, Wiktionary is NOT a dictionary. It's a crowd-sourced site and suffers from the same reliability issues as a reference as Wikipedia or Wikia. If your only definition source is unsourced Wiki dictionary data, it's NOT "general reference".
"So get a better dictionary."
As note above, none of the ones I was aware of contained the term. If it's not in M-W AND not in dictionary.com, it's not "General Reference". I don't think OED is on the web.
The use of the word relating to Christian saints is older than Tolkien, so your hypothesis is evidently wrong, and the word is not specifically related to SF.
First, Google search results indicate the opposite. The only hits on first 3 pages NOT related to Tolkien were for a Latin name of the book.
Second, the link used by the comment to illustrate its points was Google Books search.
First, a Google Books search showing some obscure book does NOT a question a "General Reference make".
Second, as usual, the only uses in books NOT related to Tolkien seemed to be NOT in English.
Based on the above, I believe the closure was completely wrong - it was DEFINITELY not general reference. It may have been argued that it was off-topic due to being more suited on English.SE, but the context makes it very SFF related IMHO.
The term, as "generally-referenced" researched, shows VERY strong correllation to being almost exclusively associated with Tolkien in English.
The only usage of the term in non-Tolkien context appears to be non-English (seemingly mostly Latin) with a strong evidence that the English version of that term is "Legendary", not "Legendarium".