Nomadic empire
The Xiongnu were a confederation of nomadic tribes from Central Asia with a ruling class of unknown origin and other subjugated tribes. They lived on the Mongolian Plateau between the 3rd century BC and the 460s AD, their territories including modern day Mongolia, southern Siberia, western Manchuria, and the modern Chinese provinces of Inner Mongolia, Gansu, and Xinjiang. The Xiongnu was the first unified empire of nomadic peoples. Relations between early Chinese dynasties and the Xiongnu were complicated and included military conflict, exchanges of tribute and trade, and marriage treaties. When Emperor Qin Shihuang drove them away from the south of the Yellow River, he built the famous Great Wall to prevent the Xiongnu from coming back.
Origin of the Huns
There are, however, potential problems with equating the names "Xiōngnú" and "Hun". Maenchen-Helfen notes that the Chinese transcription is only an approximation of their actual name. As proof of its imprecision, he notes that Emperor Wang Mang actually renamed the Xiongnu as the Hsiang-nu, with the first element meaning "to submit", while on bad terms with the group. Later, when on good terms, he renamed them Kung-nu, with the first element meaning "respectful". Christopher Beckwith, moreover, notes that the pronunciation of Xiongnu in Old Chinese is uncertain. While it was likely pronounced "*χoŋnʊ" or "*χʲoŋnʊ" in Middle Chinese, it is possible it was taken into Old Chinese before the change of initial "s" to "χ", meaning it could correspond to an Iranian name like Saka or "skuδa" (Scythian). E.G. Pulleyblank suggested an Old Chinese pronunciation similar to "*flông-nah" and connected this name to a nomadic people attested in Greek as the Phrounoi ("Φροῦνοι").