Babylon is a formable nation in the Middle East that can be formed by nations with Babylonian primary culture or Chaldean state religion, representing a revival of the ancient Babylonian state that was long the center of Mesopotamian culture and religion and conquered by the Achaemenid Empire in 539 BC. Tylos, as the only country that begins with Babylonian primary culture, is the most common candidate for reforming Babylon, though theoretically any country can form Babylon if it converts to the Chaldean religion. As a tier 2 formable, forming Babylon requires conquering many of the major cities of historical Babylonia and the Elymais - controlled by the Seleukid Empire at the start of the game - and gives claims to all of Mesopotamia, directing a resurgent Babylon to restore its historical hegemony over the region.
Compared to forming 20px Assyria, which all Aramaic culture group countries may also do if they move their capital to the region of Assyria, Babylon has somewhat more requirements (though both will involve taking down the powerful Seleukid Empire in any case) and will convert the country to Chaldean (which may or may not be desirable), but gives stronger bonuses to the capital, slightly more extensive claims, and an additional strong temporary country modifier.
Babylon the great, that most ancient of cities, which reigns over the kings of the earth; it is our destiny to break the yoke of our oppressors, and restore her to her people.