Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. Australia is the largest country by area in Oceania and the world's sixth-largest country. Australia is the oldest, flattest, and driest inhabited continent, with the least fertile soils. It is a megadiverse country, and its size gives it a wide variety of landscapes and climates, with deserts in the centre, tropical rainforests in the north-east, tropical savannas in the north, and mountain ranges in the south-east.

The ancestors of Aboriginal Australians began arriving from south-east Asia approximately 65,000 years ago, during the last ice age. Arriving by sea, they settled the continent and had formed approximately 250 distinct language groups by the time of European settlement, maintaining some of the longest known continuing artistic and religious traditions in the world. Australia's written history commenced with the European maritime exploration of Australia. The Dutch navigator Willem Janszoon was the first known European to reach Australia, in 1606. In 1770, the British explorer James Cook mapped and claimed the east coast of Australia for Great Britain, and the First Fleet of British ships arrived at Sydney in 1788 to establish the penal colony of New South Wales. The European population grew in subsequent decades, and by the end of the 1850s gold rush, most of the continent had been explored by European settlers and an additional five self-governing British colonies established. Democratic parliaments were gradually established through the 19th century, culminating with a vote for the federation of the six colonies and foundation of the Commonwealth of Australia on 1 January 1901. This began a process of increasing autonomy from the United Kingdom, highlighted by the Statute of Westminster Adoption Act 1942, and culminating in the Australia Act 1986.

Australia is a federal parliamentary constitutional monarchy, comprising six states and ten territories. Australia's population of nearly 26 million is highly urbanised and heavily concentrated on the eastern seaboard. Canberra is the nation's capital, while its most populous city and financial centre is Sydney.The next four largest cities are Melbourne, Brisbane, Perth, and Adelaide. It is ethnically diverse and multicultural, the product of large-scale immigration, with almost half of the population having one parent born overseas. Australia's abundant natural resources and well-developed international trade relations are crucial to the country's economy, which generates its income from various sources including services, mining exports, banking, manufacturing, agriculture and international education. Australia ranks amongst the highest in the world for quality of life, health, education, economic freedom, civil liberties and political rights.

Australia has a highly developed market economy and one of the highest per capita incomes globally. Australia is a regional power, and has the world's thirteenth-highest military expenditure. It is a member of international groupings including the United Nations; the G20; the OECD; the World Trade Organization; Asia-Pacific Economic Cooperation; the Pacific Islands Forum; the Pacific Community the Commonwealth of Nations; and the defence/security organisations ANZUS, AUKUS, the Five Eyes and the Quadrilateral Security Dialogue. It is a major non-NATO ally of the United States.