Maritime Southeast Asia is the maritime region of Southeast Asia as opposed to mainland Southeast Asia and comprises what is now Malaysia, Brunei, Philippines, Singapore, Indonesia, and Timor Leste. Maritime Southeast Asia is sometimes also referred to as "island Southeast Asia" or "insular Southeast Asia". The 16th century term East Indies, and the later 19th-century term Malay Archipelago refers to a largely similar area.
This main demographic difference that sets Maritime Southeast Asia apart from Indochina is that its population predominantly belongs to the Austronesian (Malayo-Polynesian, Melanesian and Micronesian) groups, although through trade with neighbouring groups from the Asian mainland like the Tai-Kadai, Austroasiatic, or Chinese, as well as other Oceanic groups like Papuans and Negritos there has been significant intermixing and cultural exchange.
The prevailing cultures of this region are very similar; maritime-based, and not majorly non-sinicized (except for Singapore, and, to a significant albeit lesser extent, Malaysia). Kingdoms based on Java and Sumatra such as Srivijaya and Majapahit spread similar cultural motifs throughout the subregions 5 countries (gong ensembles such as gamelan and kulintang). Maritime Southeast Asia makes up the oldest bloc within Austronesia, with the Philippine archipelago representing the urheimat of all Malayo-Polynesians (non-Formosan Austronesians).
Over 540 million people live in the region[when?], with the most populated island being Java. The people living there are predominantly from Austronesian subgroupings and correspondingly speak western Malayo-Polynesian languages. This region of Southeast Asia shares social and cultural ties with the peoples of mainland Southeast Asia and with other Austronesian peoples in the Pacific. Islam is the predominant religion, with Christianity being the dominant religion in the Philippines and Timor Leste. Buddhism, Hinduism, and traditional Animism are also practiced among large populations.
Historically, the region has been referred to as part of Greater India, as seen in Coedes' Indianized States of Southeast Asia, which refers to it as "Island Southeast Asia"; and within Austronesia or Oceania, due to shared ethnolinguistic and historical origins of the latter groups (Micronesian and Polynesian groups) being from this region.