In that latter sense, esp. the writings of J. R. R. Tolkien about the Middle-earth of The Lord of the Rings -- such as were first published (assuming you don't count the appendices) in The Silmarillion and continued in the volumes of The History of Middle-earth. All the background material. The original sense, which is highly specialized Church jargon, was tentatively adopted in the 19th century.
I should have run this yesterday, which was Tolkien Reading Day, but I didn't realize it till after I'd already posted yesterday's word.
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