Guanzhong (Chinese: 关中, formerly romanised as Kwanchung) region, also known as the Guanzhong Basin, Wei River Basin, or uncommonly as the Shaanzhong region, is a historical region of China corresponding to the crescentic graben basin within present-day central Shaanxi, bounded between the Qinling Mountains in the south (known as Guanzhong's "South Mountains"), and the Huanglong Mountain, Meridian Ridge and Long Mountain ranges in the north (collectively known as its "North Mountains"). The central flatland area of the basin, known as the Guanzhong Plain (关中平原; pinyin: Guānzhōng Píngyuán), is made up of alluvial plains along the lower Wei River and its numerous tributaries and thus also called the Wei River Plain. The region is part of the Jin-Shaan Basin Belt, a prominent section of the Shanxi Rift System, and is separated from its geological sibling — the Yuncheng Basin to its northeast — by the Yellow River section southwest of the Lüliang Mountains and north of the river's bend at the tri-provincial junction among Shaanxi, Shanxi and Henan.
The name Guanzhong means "within the passes", referring to the four major mountain pass fortresses historically defending the region. The region was the traditional heartland of Qin state during Zhou dynasty and thus often nicknamed the "800 li of Qin land". The Yellow River, Lüliang Mountains and the eastern end of the Qinling separate the region from the (then) politically orthodox Central Plain, which is located east of the strategic Hangu Pass and therefore was historically referred as the Guandong ("east of the pass") region by the Qin people, who later conquered the eastern states and unified China as a centralized empire — the Qin dynasty — for the first time during the 3rd century BC. Afterwards, subsequent prominent dynasties such as the Han and Tang (both considered China's historical golden ages) also had the crownland established in the Guanzhong region.