The term tribe is used in many different contexts to refer to a category of human social group. The predominant worldwide usage of the term in English is in the discipline of anthropology. Its definition is contested, in part due to conflicting theoretical understandings of social and kinship structures, and also reflecting the problematic application of this concept to extremely diverse human societies. The concept is often contrasted by anthropologists with other social and kinship groups, being hierarchically larger than a lineage or clan, but smaller than a chiefdom, nation or state. These terms are equally disputed. In some cases tribes have legal recognition and some degree of political autonomy from national or federal government, but this legalistic usage of the term may conflict with anthropological definitions. In the United States, Native American tribes are legally considered to have "domestic dependent nation" status within the territorial United States, with a government-to-government relationship with the federal government. Etymology The modern English word tribe stems from Middle English tribu, which ultimately derives from Latin tribus. According to the Oxford English Dictionary, it remains unclear if this form is the result of a borrowing from a Romance language source (such as Old French tribu) or if the form is a result of borrowing directly from Latin (the Middle English plural tribuz 1250 may be a direct representation of Latin plural tribūs). Modern English tribe may also be a result of a common pattern wherein English borrows nouns directly from Latin and drops suffixes, including -us. Latin tribus is generally held by linguists to be a compound formed from two elements: tri- 'three' and bhu, bu, fu, a verbal root meaning 'to be'. Latin tribus is held to derive from the Proto-Indo-European compound *tri-dʰh₁u/o- ('rendered in three, tripartite division'; compare with Umbrian trifu 'trinity, district', Sanskrit trídha 'threefold'). Classification Considerable debate has accompanied efforts to define and characterize tribes. In the popular imagination, tribes reflect a primordial social structure from which all subsequent civilizations and states developed. Anthropologist Elman Service presented a system of classification for societies in all human cultures, based on the evolution of social inequality and the role of the state. This system of classification contains four categories: Hunter-gatherer bands that are generally egalitarian Tribal societies with some limited instances of social rank and prestige Stratified tribal societies led by chieftains (see Chiefdom) Civilizations, with complex social hierarchies and organized, institutional