The Republic of Indonesia (Indonesian: Republik Indonesia), is a country in Southeast Asia and Oceania. Indonesia comprises 17,508 islands, and with an estimated population of around 237 million people, it is the world's fourth most populous country, and has the largest Muslim population in the world.

Indonesia is a republic, with an elected legislature and president. The nation's capital city is Jakarta. The transcontinental country shares land borders with Papua New Guinea, East Timor and Malaysia. Other neighboring countries include Singapore, Philippines, Australia, and the Indian territory of the Andaman and Nicobar Islands.

The Indonesian archipelago has been an important trade region since at least the seventh century, when the Srivijaya Kingdom traded with China and India. Local rulers gradually adopted Indian cultural, religious and political models from the early centuries CE, and Hindu and Buddhist kingdoms flourished. Indonesian history has been influenced by foreign powers drawn to its natural resources. Muslim traders brought Islam, and European powers fought one another to monopolize trade in the Spice Islands of Maluku during the Age of Discovery. Following three and a half centuries of Dutch colonialism, Indonesia secured its independence after World War II. Indonesia's history has since been turbulent, with challenges posed by natural disasters, corruption, separatism, a democratization process, and periods of rapid economic change.

Across its many islands, Indonesia consists of distinct ethnic, linguistic, and religious groups. The Javanese are the largest and most politically dominant ethnic group. Indonesia has developed a shared identity defined by a national language, ethnic diversity, religious pluralism within a majority Muslim population, and a history of colonialism and rebellion against it. Indonesia's national motto, "Bhinneka Tunggal Ika" ("Unity in Diversity" literally, "many, yet one"), articulates the diversity that shapes the country. However, sectarian tensions and separatism have led to violent confrontations that have undermined political and economic stability. Despite its large population and densely populated regions, Indonesia has vast areas of wilderness that support the world's second highest level of biodiversity. The country is richly endowed with natural resources, yet poverty is a defining feature of contemporary Indonesia.

The name Indonesia derives from the Latin Indus, meaning "India", and the Greek nesos, meaning "island". The name dates to the 18th century, far predating the formation of independent Indonesia. In 1850, George Earl, an English ethnologist, proposed the terms Indunesians — and, his preference, Malayunesians — for the inhabitants of the "Indian Archipelago or Malayan Archipelago". In the same publication, a student of Earl's, James Richardson Logan, used Indonesia as a synonym for Indian Archipelago. However, Dutch academics writing in East Indies publications were reluctant to use Indonesia. Instead, they used the terms Malay Archipelago (Maleische Archipel); the Netherlands East Indies (Nederlandsch Oost Indië), popularly Indië; the East (de Oost); and even Insulinde.

From 1900, the name Indonesia became more common in academic circles outside the Netherlands, and Indonesian nationalist groups adopted it for political expression. Adolf Bastian, of the University of Berlin, popularized the name through his book Indonesien oder die Inseln des Malayischen Archipels, 1884–1894. The first Indonesian scholar to use the name was Suwardi Suryaningrat (Ki Hajar Dewantara), when he established a press bureau in the Netherlands with the name Indonesisch Pers-bureau in 1913.

Indonesia consists of 17,508 islands, about 6,000 of which are inhabited. These are scattered over both sides of the equator. The five largest islands are Java, Sumatra, Kalimantan (the Indonesian part of Borneo), New Guinea (shared with Papua New Guinea), and Sulawesi. Indonesia shares land borders with Malaysia on the islands of Borneo and Sebatik, Papua New Guinea on the island of New Guinea, and East Timor on the island of Timor. Indonesia also shares borders with Singapore, Malaysia, and the Philippines to the north and Australia to the south across narrow straits of water. The capital, Jakarta, is on Java and is the nation's largest city, followed by Surabaya, Bandung, Medan, and Semarang.

At 1,919,440 square kilometers (741,050 sq mi), Indonesia is the world's 16th-largest country in terms of land area. Its average population density is 134 people per square kilometer (347 per sq mi), 79th in the world, although Java, the world's most populous island, has a population density of 940 people per square kilometer (2,435 per sq mi). At 4,884 meters (16,024 ft), Puncak Jaya in Papua is Indonesia's highest peak, and Lake Toba in Sumatra its largest lake, with an area of 1,145 square kilometers (442 sq mi). The country's largest rivers are in Kalimantan, and include the Mahakam and Barito; such rivers are communication and transport links between the island's river settlements.

Indonesia's location on the edges of the Pacific, Eurasian, and Australian tectonic plates makes it the site of numerous volcanoes and frequent earthquakes. Indonesia has at least 150 active volcanoes, including Krakatoa and Tambora, both famous for their devastating eruptions in the 19th century. The eruption of the Toba supervolcano, approximately 70,000 years ago, was one of the largest eruptions ever, and a global catastrophe. Recent disasters due to seismic activity include the 2004 tsunami that killed an estimated 167,736 in northern Sumatra,[66] and the Yogyakarta earthquake in 2006. However, volcanic ash is a major contributor to the high agricultural fertility that has historically sustained the high population densities of Java and Bali.

Lying along the equator, Indonesia has a tropical climate, with two distinct monsoonal wet and dry seasons. Average annual rainfall in the lowlands varies from 1,780–3,175 millimeters (70–125 in), and up to 6,100 millimeters (240 in) in mountainous regions. Mountainous areas—particularly in the west coast of Sumatra, West Java, Kalimantan, Sulawesi, and Papua—receive the highest rainfall. Humidity is generally high, averaging about 80%. Temperatures vary little throughout the year; the average daily temperature range of Jakarta is 26–30 °C (79–86 °F).

Indonesia's size, tropical climate, and archipelagic geography, support the world's second highest level of biodiversity (after Brazil), and its flora and fauna is a mixture of Asian and Australasian species. Once linked to the Asian mainland, the islands of the Sunda Shelf (Sumatra, Java, Borneo, and Bali) have a wealth of Asian fauna. Large species such as the tiger, rhinoceros, orangutan, elephant, and leopard, were once abundant as far east as Bali, but numbers and distribution have dwindled drastically. Forests cover approximately 60% of the country. In Sumatra and Kalimantan, these are predominantly of Asian species. However, the forests of the smaller, and more densely populated Java, have largely been removed for human habitation and agriculture. Sulawesi, Nusa Tenggara, and Maluku—having been long separated from the continental landmasses—have developed their own unique flora and fauna. Papua was part of the Australian landmass, and is home to a unique fauna and flora closely related to that of Australia, including over 600 bird species.

Indonesia is second only to Australia in its degree of endemism, with 26% of its 1,531 species of bird and 39% of its 515 species of mammal being endemic. Indonesia's 80,000 kilometers (50,000 mi) of coastline are surrounded by tropical seas that contribute to the country's high level of biodiversity. Indonesia has a range of sea and coastal ecosystems, including beaches, sand dunes, estuaries, mangroves, coral reefs, sea grass beds, coastal mudflats, tidal flats, algal beds, and small island ecosystems. The British naturalist, Alfred Wallace, described a dividing line between the distribution and peace of Indonesia's Asian and Australasian species. Known as the Wallace Line, it runs roughly north-south along the edge of the Sunda Shelf, between Kalimantan and Sulawesi, and along the deep Lombok Strait, between Lombok and Bali. West of the line the flora and fauna are more Asian; moving east from Lombok, they are increasingly Australian. In his 1869 book, The Malay Archipelago, Wallace described numerous species unique to the area. The region of islands between his line and New Guinea is now termed Wallacea. 

Indonesia's high population and rapid industrialization present serious environmental issues, which are often given a lower priority due to high poverty levels and weak, under-resourced governance. Issues include large-scale deforestation (much of it illegal) and related wildfires causing heavy smog over parts of western Indonesia, Malaysia and Singapore; over-exploitation of marine resources; and environmental problems associated with rapid urbanization and economic development, including air pollution, traffic congestion, garbage management, and reliable water and waste water services. Habitat destruction threatens the survival of indigenous and endemic species, including 140 species of mammals identified by the World Conservation Union (IUCN) as threatened, and 15 identified as critically endangered, including the Sumatran Orangutan.

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