The term 'River Constellation' refers to the Eridanus constellation, which is an ancient constellation that was first catalogued by the Greek astronomer Ptolemy in the 2nd century. It is one of the largest constellations in the night sky, stretching over a large area of the southern celestial hemisphere. The constellation is named after the mythological river Eridanos, which was said to flow from the east to the west. The constellation contains several stars, including Achernar, the brightest star in the constellation, and several deep-sky objects, such as the Eridanus Supervoid, the largest known void in the universe.