When people refer to the "Chengdu basin," they are usually talking about the Chengdu Plain (also known as the Chuanxi Plain), which sits on the westernmost edge of the massive Sichuan Basin.
The Sichuan Basin is a giant, bowl-like lowland in southwestern China surrounded by high mountain ranges, including the Tibetan Plateau to the west. Because the basin's soil is rich in red sandstone and purple shale, it is historically nicknamed the "Red Basin".
The Chengdu Plain itself is a highly fertile, flat alluvial plain of about 6,000 square kilometers. It was formed over millennia by sediment washed down from the mountains by the Min River and other waterways. Thanks to the ancient Dujiangyan Irrigation System built in 256 BCE, the plain has been protected from floods and perfectly irrigated for over 2,000 years, earning Chengdu the nickname "The Land of Abundance".