The Asteroid Belt, also known as the Main Asteroid Belt as means to distinguish it from the Kuiper Belt, often referred as just "The Belt", is a circumstellar disc located in the the region of the Sol system between the orbits of the planets Mars and Jupiter. It consists of numerous asteroids and is part of the "outer planets" political region.
The Belt was colonized by humans roughly 150 years before the events of Leviathan Wakes. The humans who are born in the Belt, called Belters, are taller and thinner than humans from the inner planets Earth and Mars due to the lower gravity, and also have larger heads.
While the population of the Belt is around 50 million according to official inner planet censuses, other sources such as the Outer Planets Alliance claim the actual figure is near 100 million.