The Great Wall of China (traditional Chinese: 萬里長城; simplified Chinese: 万里长城; pinyin: Wànlǐ Chángchéng, literally "ten thousand li long wall") is a series of fortifications in China. They were built across the historical northern borders of ancient Chinese states and Imperial China as protection against various nomadic groups from the Eurasian Steppe. The first walls date to the 7th century BC; these were joined together in the Qin dynasty. Successive dynasties expanded the wall system; the best-known sections were built by the Ming dynasty (1368–1644).

To aid in defense, the Great Wall utilized watchtowers, troop barracks, garrison stations, signaling capabilities through the means of smoke or fire, and its status as a transportation corridor. Other purposes of the Great Wall have included border controls (allowing control of immigration and emigration, and the imposition of duties on goods transported along the Silk Road), and the regulation of trade.

The collective fortifications constituting the Great Wall stretch from Liaodong in the east to Lop Lake in the west, and from the present-day Sino–Russian border in the north to Tao River in the south: an arc that roughly delineates the edge of the Mongolian steppe, spanning 21,196.18 km (13,170.70 mi) in total. It is a UNESCO World Heritage Site, and was voted one of the New 7 Wonders of the World in 2007. Today, the defensive system of the Great Wall is recognized as one of the most impressive architectural feats in history.

The collection of fortifications known as the Great Wall of China has historically had a number of different names in both Chinese and English.

In Chinese histories, the term "Long Wall(s)" (t 長城, s 长城, Chángchéng) appears in Sima Qian's Records of the Grand Historian, where it referred both to the separate great walls built between and north of the Warring States and to the more unified construction of the First Emperor. The Chinese character 城, meaning city or fortress, is a phono-semantic compound of the "earth" radical 土 and phonetic 成, whose Old Chinese pronunciation has been reconstructed as *deŋ. It originally referred to the rampart which surrounded traditional Chinese cities and was used by extension for these walls around their respective states; today, however, it is more commonly used to mean "city".

The longer Chinese name "Ten-Thousand Mile Long Wall" (t 萬里長城, s 万里长城, Wànlǐ Chángchéng) came from Sima Qian's description of it in the Records, though he did not name the walls themselves as such. The AD 493 Book of Song quotes the frontier general Tan Daoji referring to "the long wall of 10,000 miles", closer to the modern name, but the name rarely features in pre-modern times otherwise. The traditional Chinese mile (里, lǐ) was an often irregular distance that was intended to show the length of a standard village and varied with terrain but was usually standardized at distances around a third of an English mile (540 m). However, this use of "ten-thousand" (wàn) is figurative in a similar manner to the Greek and English myriad and simply means "innumerable" or "immeasurable".

Because of the wall's association with the First Emperor's supposed tyranny, the Chinese dynasties after Qin usually avoided referring to their own additions to the wall by the name "Long Wall". Instead, various terms were used in medieval records, including "frontier(s)" (塞, Sài), "rampart(s)" (垣, Yuán), "barrier(s)" (障, Zhàng), "the outer fortresses" (外堡, Wàibǎo), and "the border wall(s)" (t 邊牆, s 边墙, Biānqiáng). Poetic and informal names for the wall included "the Purple Frontier" (紫塞, Zǐsài) and "the Earth Dragon" (t 土龍, s 土龙, Tǔlóng). Only during the Qing period did "Long Wall" become the catch-all term to refer to the many border walls regardless of their location or dynastic origin, equivalent to the English "Great Wall".

Sections of the wall in south Gobi Desert and Mongolian steppe are sometimes referred to as "Wall of Genghis Khan", even though Genghis Khan did not construct any walls or permanent defense lines himself.