From which Old English term did J.R.R. Tolkien derive the name Middle-earth?

J.R.R. Tolkien derived the name Middle-earth from the Old English term middangeard.

Middangeard referred to the inhabited world or human realm in Old English tradition. Tolkien adapted the concept and its modern English form, “Middle-earth,” for the lands where most of the events in The Hobbit and The Lord of the Rings occur. The term therefore has a historical and linguistic background rather than being an entirely invented label.

Tolkien’s use of the name is also connected with related terms in other Germanic languages, including Old Norse miðgarðr. These words describe a world positioned between or among other realms. Tolkien’s fictional Middle-earth is not the whole universe; it is a region of Arda, the larger world of his mythology.

A frequent misconception is that Middle-earth means a planet separate from Earth. Tolkien instead presented his stories as belonging to an imagined ancient history of our own world, though the chronology and geography are fictional.