Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia or SEA, is the geographical southeastern subregion of Asia, consisting of the regions that are south of China, south-east of the Indian subcontinent and north-west of Australia. Southeast Asia is bordered to the north by East Asia, to the west by South Asia and the Bay of Bengal, to the east by Oceania and the Pacific Ocean, and to the south by Australia and the Indian Ocean. Apart from the British Indian Ocean Territory and two out of 26 atolls of Maldives in South Asia, Southeast Asia is the only other subregion of Asia that lies partly within the Southern Hemisphere. The majority of the subregion is still in the Northern Hemisphere. East Timor and the southern portion of Indonesia are the only parts that are south of the Equator.
In contemporary definition, Southeast Asia consists of two cultural regions:
Mainland Southeast Asia, also known as the Indochinese Peninsula, comprising Cambodia, Laos, Myanmar, Peninsular Malaysia, Thailand, and Vietnam.
Maritime Southeast Asia, mostly consisting of the Malay Archipelago, comprising the Andaman and Nicobar Islands (India), Brunei, Christmas Island (Australia), the Cocos (Keeling) Islands (Australia), East Malaysia, East Timor, Indonesia, the Philippines, and Singapore.
The region lies near the intersection of geological plates, with both heavy seismic and volcanic activities. The Sunda Plate is the main plate of the region, featuring almost all Southeast Asian countries except Myanmar, northern Thailand, northern Laos, northern Vietnam, and northern Luzon of the Philippines. The mountain ranges in Myanmar, Thailand, and Peninsular Malaysia are part of the Alpide belt, while the islands of the Philippines are part of the Pacific Ring of Fire. Both seismic belts meet in Indonesia, causing the region to have relatively high occurrences of earthquakes and volcanic eruptions.
most populous geographical region in Asia after South Asia and East Asia. The region is culturally and ethnically diverse, with hundreds of languages spoken by different ethnic groups. Ten countries in the region are members of the Association of Southeast Asian Nations (ASEAN), a regional organization established for economic, political, military, educational and cultural integration amongst its members, Southeast Asia is geographically divided into two subregions, namely Mainland Southeast Asia (or the Indochinese Peninsula) and Maritime Southeast Asia (or the similarly defined Malay Archipelago) (Javanese: Nusantara).
Mainland Southeast Asia includes:
Cambodia
Laos
Myanmar (Burma)
Peninsular Malaysia
Thailand
Vietnam
Maritime Southeast Asia includes:
Brunei
East Malaysia
East Timor
Indonesia
Philippines
Singapore
Although Peninsular Malaysia is geographically situated in Mainland Southeast Asia, it also shares many similar cultural and ecological affinities with surrounding islands, thus it serves as a bridge of two subregions. Geographically, the Andaman and Nicobar Islands of India is also considered a part of Maritime Southeast Asia. Eastern Bangladesh and Northeast India have strong cultural ties with Mainland Southeast Asia and are sometimes considered transregional areas between South Asia and Southeast Asia. To the east, Hong Kong is sometimes regarded as part of Southeast Asia. Similarly, Christmas Island and the Cocos (Keeling) Islands have strong cultural ties with Maritime Southeast Asia and are sometimes considered transregional areas between Southeast Asia and Australia/Oceania. On some occasions, Sri Lanka has been considered a part of Southeast Asia because of its cultural and religious ties to Mainland Southeast Asia. The eastern half of the island of New Guinea, which is not a part of Indonesia, namely, Papua New Guinea, is sometimes included as a part of Maritime Southeast Asia, and so are Guam, the Northern Mariana Islands, and Palau, which were all parts of the Spanish East Indies with strong cultural and linguistic ties to the region, specifically, the Philippines.
East Timor and the eastern half of Indonesia (east of the Wallace Line in the region of Wallacea) are considered to be geographically associated with Oceania due to their distinctive faunal features. Geologically, the island of New Guinea and its surrounding islands are considered as parts of the Australian continent, connected via the Sahul Shelf. Both Christmas Island and the Cocos (Keeling) Islands are located on the Australian Plate, south of the Java Trench. Even though they are geographically closer to Maritime Southeast Asia than mainland Australia, these two Australian external territories are not geologically associated with Asia as none of them is actually on the Sunda Plate. The United Nations geoscheme has classified both island territories as parts of Oceania, under the Australia and New Zealand (Australasia) subregion.