Xengiman is an Axunašin term, literally meaning the "Greater Xengi"; it makes a convenient label for the great southern plain that is the home to Xurno and ancient Axunai as well as most of their neighbors. As the name suggests, Xengiman is normally dominated by the culture that controls the Xengi valley. It thus parallels Eretald for the northern plain on which Verduria lies.
The Axunemi would have included Čeiy in the term, and it's useful to consider Čeiy part of Xengiman in ancient times, but not in modern times when Čeiy no longer strongly identifies with Xurnese culture.
Broadly speaking Xengiman consists of the watershed of the entire Xengi system (including the Bonari, the Doju, and the Puro), plus that of the smaller rivers to the west, the Ideis, the Nineye, and the Čiqay. "Xengi plain" sounds like it excludes the latter areas, while "Xurnese plain" seems misleading, as well as inaccurate for the ancient era. There are various significant attractions for tourists in Xengiman.
Note that "Xengi valley" is a narrower term, referring to the Xengi alone.