The Void was an area beyond Arda and the world, which may have been the equivalent to space or the universe.
According to the Ainulindalë, Ilúvatar, the All-father, emerged out of the Void and created the Timeless Halls, which seems to be separate from the Void. Before the creation of Eä, Melkor searched this space thoroughly for the Flame Imperishable, the source of Eru's creative power, but was unsuccessful. It is believed that many creatures evolved in the Void due to the discords in the Music of the Ainur caused by Melkor. Ungoliant may even have been one of these creatures. Later, after his defeat in the War of Wrath, Morgoth was cast out into the Void, but legends predict that he will return to the World before its end in Dagor Dagorath (Last Battle).
The Void was also known as the Outer Void, the Timeless Void, the Everlasting Dark, the Ancient Darkness, and the Outside.