In Latin, the term terra nullius means "land belonging to nobody." It does not, however, seem to have been a Roman concept. Not being great discoverers, the Romans had to acquire their empire the old-fashioned way: they fought for it.
Starting in the 17th century, terra nullius denoted a legal concept allowing a European colonial power to take control of "empty" territory that none of the other European colonial powers had claimed.
Of course, most of these "empty" territories were inhabited, so the meaning of terra nullius grew to include territories considered "devoid of civilized society." The most celebrated example is that of Australia, where the concept of terra nullius still features in lawsuits pressed by the Aboriginal peoples. Other examples of lands once considered terra nullius would be Siberia and the Americas. The United States seems to have treated its Western frontier as terra nullius in the rush to fulfill its perceived Manifest Destiny.
Terra nullius in Australia
The United Kingdom relied on this principle to claim possession of the Australian continent. It was deemed that, prior to the arrival of Europeans, Australia was "a tract of territory practically unoccupied, without settled inhabitants or settled law" (as the Privy Council put it in 1889). This was, of course, a legal fiction, as the continent was inhabited by native peoples – the Australian Aborigines – and legal codes were already operative in some places, as in the case of the Aborigines of the Yirrkala mission. This was overlooked or ignored by the colonial authorities. In this regard, the application of terra nullius to Australia was inconsistent with the practice elsewhere in the British Empire. The British Government attempted as early as the 1830s and 1840s to bring Australian colonial practice into line with international law as it then stood, and with the approach taken elsewhere in the Empire. Its efforts were unsuccessful and Australian policy...