Iani Chaos is a region of chaos terrain at the south end of the outflow channel Ares Vallis, of the Margaritifer Sinus quadrangle region of the planet Mars, centered at approximately ~342°E, 2°S. The chaotic terrain is widely believed to have formed via the removal of subsurface water or ice, resulting in flooding at the surface, and the formation of Ares Vallis. Within Iani Chaos, deposited stratigraphically above the chaotic terrain, are smooth, low-slope, intermediate-to-light-toned deposits that are rich in a hydrated mineral that is most likely gypsum as well as hematite. (wikipedia entry)
A chaos is a distinctive area of broken or jumbled terrain. On Mars they are named after the nearest named albedo feature on the Schiaparelli or Antoniadi maps.