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.

Mainland Australia is the main landmass of the Australian continent, excluding the Aru Islands, New Guinea, Tasmania, and other Australian offshore islands. The landmass also constitutes the mainland of the territory governed by the Commonwealth of Australia, and the term, along with continental Australia, can be used in a geographic sense to exclude surrounding continental islands and external territories. Generally, the term is applied to the states of New South Wales, Queensland, South Australia, Victoria, and Western Australia, as well as the Australian Capital Territory, Jervis Bay Territory, and Northern Territory.