Angkor is a name conventionally applied to the region of Cambodia serving as the seat of the Khmer empire that flourished from approximately the 9th century to the 15th century A.D.
(The word "Angkor" itself is derived from the Sanskrit "nagara," meaning "city.")More precisely, the Angkorian period may be defined as the period from 802 A.D., when the Khmer Hindu monarch Jayavarman II declared himself the "universal monarch" and "god-king" of Cambodia, until 1431 A.D., when Thai invaders sacked the Khmer capital, causing its population to migrate south to the area of Phnom Penh.

The ruins of Angkor are located amid forests and farmland to the north of the Great Lake
(Tonle Sap) and south of the Kulen Hills, near modern day Siem Reap, and are a UNESCO World Heritage Site. The temples of the Angkor area number over one thousand, ranging in scale from nondescript piles of brick rubble scattered through rice fields to the magnificent Angkor Wat,
said to be the world's largest single religious monument. Many of the temples at Angkor have been restored, and together they comprise the most significant site of Khmer architecture.
Visitor numbers approach two million annually.

In 2007 an international team of researchers using satellite photographs and other modern techniques concluded that Angkor had been the largest preindustrial city in the world with an urban sprawl of 400 square miles. The closest rival to Angkor, the Mayan city of Tikal in Guatemala, was roughly 50 square miles in total size.