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, ethnicity, nation or state. These terms are similarly 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.

Nomads are communities without fixed habitation who regularly move to and from areas. Such groups include hunter-gatherers, pastoral nomads, tinkers and trader nomads. In the twentieth century, the population of nomadic pastoral tribes slowly decreased, reaching an estimated 30–40 million nomads in the world as of 1995.