Southeast Asia is a subregion of Asia, consisting of the countries that are geographically south of China, east of India, west of New Guinea and north of Australia.^{[citation needed]} The region lies on the intersection of geological plates, with heavy seismic and volcanic activity. Southeast Asia consists of two geographic regions: Mainland Southeast Asia, also known as Indochina, comprises Cambodia, Laos, Burma (Myanmar), Thailand, Vietnam and Peninsular Malaysia, and Maritime Southeast Asia, which is analogous to the Malay Archipelago, comprises Brunei, East Malaysia, East Timor, Indonesia, Philippines, Christmas Island, and Singapore^{[citation needed]}.^{[dead link]}

Austronesian peoples predominate in this region. The major religions are Islam and Buddhism, followed by Christianity. However, a wide variety of religions are found throughout the region, including many Hindu and animist-influenced practices.

Definitions of "Southeast Asia" vary, but most definitions include the area represented by the countries and territories listed below. All of the countries excluding East Timor are members of the Association of Southeast Asian Nations (ASEAN). The area, together with part of South Asia, was widely known as the East Indies or simply the Indies until the 20th century. Christmas Island and the Cocos (Keeling) Islands are considered part of Southeast Asia though they are governed by Australia. Sovereignty issues exist over some islands in the South China Sea. Papua New Guinea has stated that it might join ASEAN, and is currently an observer.

East Asia or Eastern Asia (the latter form preferred by the United Nations) is a subregion of Asia that can be defined in either geographical or cultural terms. Geographically and geo-politically, it covers about 12,000,000 km² (4,600,000 sq mi), or about 28 percent of the Asian continent, about 15 percent bigger than the area of Europe.

More than 1.5 billion people, about 38% of the population of Asia or 22% of all the people in the world, live in geographic East Asia, about twice Europe's population. The region is one of the world's most populated places, with a population density of 133 inhabitants per square kilometre (340 /sq mi), being about three times the world average of 45 /km² (120 /sq mi), although Mongolia has the lowest population density of a sovereign state. Using the UN subregion definitions, it ranks second in population only to Southern Asia.

Historically, many societies in East Asia have been part of the Chinese cultural sphere, and East Asian vocabulary and scripts are often derived from Classical Chinese and Chinese script. Sometimes Northeast Asia is used to denote Japan and Korea. Major religions include Buddhism (mostly Mahayana), Confucianism or Neo-Confucianism, Taoism, Chinese folk religion in China, Shinto in Japan, Shamanism in Korea, Mongolia and other indigenous populations of northern East Asia, and recently Christianity in South Korea. The Chinese Calendar is the root from which many other East Asian calendars are derived.

Southeast or south east is the ordinal direction halfway between east and south, the opposite of northwest.

Southeast, south east, or Southeastern may also refer to: