The English name of the main setting in most of Tolkien's mythology. In Danish, it's usually written as Midgård, the land of humans in Norse Mythology (from which Tolkien drew heavy inspiration). In Quenya Elvish, it's named Endor, which coincides with the name of a certain forest moon in Star Wars. Although "Middle-Earth" refers to a single continent in Tolkien's world, the name is also used to refer to the entire world, including the Undying Lands (where immortals, such as elves and Ainur, live), the lost island kingdom of Númenor.
Tolkien intended the world as a kind of distant mythological past of our own world, and drew heavy inspiration from various ancient mythologies he knew about. For instance, Númenor is clearly inspired by the Atlantis myths (in fact, it is named "Atalantë" in Eldarin Elvish), and many of the creatures that inhabit Middle-Earth are lifted directly out of Norse and Anglo-Saxon myth. Others (such as orcs) are based on composites of several mythological creatures.