According to Wikipedia, an erg is a "large, relatively flat area of desert covered with wind-swept sand with little to no vegetation cover. Strictly speaking, an erg is defined to be a desert area that contains more than 125 square kilometers of eolian sand and where sand covers more than 20% of the surface... The largest hot desert in the world, the Sahara, is 9,000,000km² and contains several ergs, such as Erg Chech in Algeria. Approximately 85% of all the Earth's mobile sand is found in ergs that are larger than 32,000 km². Ergs are also found on other celestial bodies, such as Venus and Saturn's moon Titan."