Oceania was originally conceived as the lands of the Pacific Ocean, stretching from the Straits of Malacca to the coast of the Americas. It comprised four regions: Polynesia, Micronesia, Malaysia (now called the Malay Archipelago), and Melanesia (now called Australasia). Included are parts of three geological continents, Eurasia, Australia, and Zealandia, as well the non-continental volcanic islands of the Philippines, Wallacea, and the open Pacific. It extends to Sumatra in the west, the Bonin Islands in the northwest, the Hawaiian Islands in the northeast, Rapa Nui and Sala y Gómez Island in the east, and Macquarie Island in the south, but excludes Taiwan, the Japanese Archipelago (including the Ryukyu Islands), and Aleutian Islands of the margins of Asia.

The term is sometimes used more specifically to denote a continent comprising Australia and proximate islands, or biogeographically as a synonym for either the Australasian ecozone (Wallacea and Australasia) or the Pacific ecozone (Melanesia, Polynesia, and Micronesia apart either from New Zealand or from mainland New Guinea).