Middle-earth is the fictional-universe setting of the majority of author J. R. R. Tolkien's fantasy writings. The Hobbit and The Lord of the Rings take place entirely in Middle-earth, as does much of The Silmarillion and Unfinished Tales. Properly, Middle-earth is the central continent of the imagined world, not a name of the entire world.
Tolkien prepared several maps of Middle-earth and of the regions of Middle-earth where his stories took place. Some were published in his lifetime, though some of the earliest maps were not published until after his death. The main maps were those published in The Hobbit, The Lord of the Rings, The Silmarillion, and Unfinished Tales. Most of the events of the First Age took place in the subcontinent Beleriand, which was later engulfed by the ocean at the end of the First Age; the Blue Mountains at the right edge of the map of Beleriand are the same Blue Mountains that appear on the extreme left of the map of Middle-earth in the Second and Third Ages. Tolkien's map of Middle-earth, however, shows only a small part of the world; most of the lands of Rhûn and Harad are not shown on the map, and there are also other continents. More: en.wikipedia.org.