Inner Asia refers to regions within East Asia and North Asia that are today part of western China, Mongolia and eastern Russia. It overlaps with some definitions of Central Asia, mostly the historical ones, but certain regions of Inner Asia (such as Northeast China) are not considered a part of Central Asia by any of its definitions. Inner Asia may be considered as the "frontier" of China, and as bounded by East Asia, which consists of China, Japan, and Korea.
The extent of Inner Asia was seen differently in different periods. "Inner Asia" is sometimes contrasted to "China Proper", that is, the original provinces, those with majority Han Chinese populations. In 1800 it consisted of four main areas, namely Manchuria (modern Northeast China and Outer Manchuria), Mongolia (Inner and Outer), Xinjiang and Tibet. These areas had been recently conquered by the Qing dynasty but were governed through different administrative structure not as regular provinces during most of the Qing period. The Qing government agency known as the Lifan Yuan was established to supervise the empire's Inner Asian regions.
Inner Asia has a range of definitions and usages. One such is how Denis Sinor used Inner Asia in contrast to agricultural civilizations, noting its changing borders, such as when a Roman province was taken by the Huns, areas of North China was occupied by the Mongols, or when Anatolia came under Turkish influence, eradicating Hellenistic culture.
The terms meaning "Inner Asia" in the languages of Inner Asian peoples are all modern loan translations of European, mostly Russian, terms.
"Central Asia" normally denotes the western, Islamic part of Inner Asia; that is, Kazakhstan, Uzbekistan, Kyrgyzstan, Turkmenistan, and Tajikistan, with Afghanistan sometimes also included as part of Central Asia. However, The Library of Congress subject classification system treats "Central Asia" and Inner Asia as synonymous.
According to Morris Rossabi, the term "Inner Asia" is the well-established term for the area in the literature. However, because of its deficiencies, including the implication of an "Outer Asia" that does not exist, Denis Sinor has proposed the neologism "Central Eurasia", which emphasizes the role of the area in intercontinental exchange. According to Sinor:
The definition that can be given of Central Eurasia in space is negative. It is that part of the continent of Eurasia that lies beyond the borders of the great sedentary civilizations.... Although the area of Central Eurasia is subject to fluctuations, the general trend is that of diminution. With the territorial growth of the sedentary civilizations, their borderline extends and offers a larger surface on which new layers of barbarians will be deposited.