Oceania is a name used in geography for the region made up of Australia, Polynesia, Melanesia, and several other island nations in the surrounding area. It is often listed as one of the continents of the world.
The term "Oceania" does not have one single agreed definition. The widest definition of Oceania includes the entire region between continental Asia and the Americas, including Australasia, as well as islands in the Pacific Rim such as the Japanese archipelago, Taiwan, and the Aleutian islands.
On the other hand, the Oceania ecozone includes all of Micronesia, Fiji, and all of Polynesia except New Zealand. Sometimes, people use the term 'Oceania' to include only the Polynesian and Melanesian islands in the Pacific Ocean, as separate from Australasia. When Oceania is treated seperately from Australia, the region of Oceania is referred to as "Australia and Oceania".
A smaller usage of Oceania as a continent includes only Australia, Tasmania, New Guinea, and the islands between them. This part of the world is usually called Australasia instead. For these reasons, it is not correct to say that Australasia is part of Oceania, because what is meant by 'Oceania' is not clear. Australasia has an exact definition in biogeography and geology; Australasia includes New Zealand, Australia (including Tasmania), and Melanesia, New Guinea, and the islands just north and east of Australia. All these are south-east of the Bali–Lombok line, also known as the Wallace Line.