What Old English word inspired Tolkien’s name Middle-earth?

The Old English word that inspired Tolkien’s name Middle-earth is middangeard.

Middangeard, also written middanġeard in normalized scholarly forms, referred to the inhabited world in Old English tradition. Tolkien adapted the concept into Middle-earth, the central region in which most of The Hobbit and The Lord of the Rings take place. The term is related to the Norse miðgarðr, or Midgard, and to similar expressions in other Germanic languages.

Middle-earth does not mean a planet separate from Earth. In Tolkien’s mythology it is part of Arda, the world, and corresponds broadly to the lands of the eastern side of the continent during the periods described in his fiction. This is why the name can sound familiar from real historical languages while functioning as a fictional geographical term. A frequent mistake is to treat Middle-earth as the entire Tolkien universe; Valinor, Númenor and other regions are also important but are not simply synonyms for Middle-earth.