noun: (chiefly Norse mythology) synonym for Midgard, the world of traditional Germanic cosmology, conceived as a realm between heaven (Asgard) and hell (Niflheim).
noun: Alternative letter-case form of Middle Earth [(archaic) The Earth]
noun: (fiction) Alternative form of Middle-earth: the setting of the Lord of the Rings series. [(fiction) The main setting of J.R.R. Tolkien's Lord of the Rings and other works.]