The region lies near the intersection of geological plates, with heavy seismic and volcanic activities. The Sunda Plate is the main plate of the region, features almost all Southeast Asian countries except Myanmar, northern Thailand, 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.
Southeast Asia covers about 4.5 million km2 (1.7 million mi2), which is 10.5% of Asia or 3% of earth's total land area. Its total population is more than 641 million, about 8.5% of world's population. It is the third most populous geographical region in the world 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 ASEAN, a regional organisation established for economic, political, military, educational and cultural integration amongst its members.
The region, together with part of South Asia, was well known by the Europeans as the East Indies or simply the Indies until the 20th century. Chinese sources referred the region as 南洋 (Nanyang), which literally means the Southern Ocean. The mainland section of Southeast Asia was referred to as Indochina by European geographers due to its location between China and the Indian subcontinent and cultural influences from both neighboring regions. In the 20th century, however, the term became more restricted to former French Indochina territory (Cambodia, Laos, and Vietnam). The maritime section of Southeast Asia is also known as Malay Archipelago, a term derived from the European concept of a Malay race. Another term for Maritime Southeast Asia is Insulindia (Indian Islands), used to describe the region between Indochina and Australasia.
The term "Southeast Asia" was first used in 1839 by an American pastor Howard Malcolm in his book entitled Travels in South-Eastern Asia. Malcolm only included the Mainland section and excluded the Maritime section in his definition of Southeast Asia. The term was officially used in the midst of World War II by the Allies, through the formation of South East Asia Command (SEAC) in 1943. SEAC popularised the use of the term "Southeast Asia", although what constituted Southeast Asia in the early days was not fixed, for example the Philippines and a large part of Indonesia were excluded by SEAC while Ceylon was included. However, by the late 1970s, a roughly standard usage of the term "Southeast Asia" and the territories it encompasses had emerged. Although from a cultural or linguistic perspective, the definitions of "Southeast Asia" may vary, the most common definitions nowadays include the area represented by the countries (sovereign states and dependent territories) listed below.
Ten of eleven states of Southeast Asia are members of the Association of Southeast Asian Nations (ASEAN), while East Timor is an observer state. Papua New Guinea has stated that it might join ASEAN, and is currently an observer. Sovereignty issues exist over some territories in the South China Sea.