Tribalism is the state of being organized by, or advocating for, tribes or tribal lifestyles. Human evolution primarily occurred in small hunter-gatherer groups, as opposed to in larger and more recently settled agricultural societies or civilizations. With a negative connotation and in a political context, tribalism can also mean discriminatory behavior or attitudes towards out-groups, based on in-group loyalty.

The term "tribe" derives from the ancient Latin word "tribus" and can be defined to mean an extended kin group or clan with a common ancestor. A tribe can also be described as a group who share the common interest of mutual survival and preservation of a common culture. The proverb "birds of a feather flock together" describes homophily, the human tendency to form friendship networks with people of similar occupations, interests, and habits. Some tribes can be located in geographically proximate areas, like villages or bands, and although telecommunications in theory could enable groups of people to form tribe-like communities, digital tribes and social networking websites are not quite tribes in that they do not inherently provide the mutual survival of both the individual members of the tribe and for the tribe itself, as tribes do.

In terms of conformity, the word "tribalism" has been defined as a "subjectivity" or "way of being" social frame in which communities are bound socially beyond immediate birth ties by the dominance of various modalities of face-to-face and object integration. Ontologically, tribalism is oriented around the valences of analogy, genealogy and mythology. That means that customary tribes have their social foundations in some variation of these tribal orientations, while often taking on traditional practices (e.g. Abrahamic religions such as Christianity, Judaism, and Islam), and modern practices, including monetary exchange, mobile communications, and modern education.

Tribalism in a political sense refers to the strong political solidarity typical of post-truth politics.

The social structure of a tribe can vary greatly from case to case. The relatively small size of customary tribes results in a social life which usually involve a relatively few significant political or economic distinctions between individuals. As a result, social hierarchy is uncommon, and deep bonds are made between individual members.

A tribe often refers to itself using its own language's word for "people", and refers to other, neighboring tribes with various words to distinguish them as other. For example, the term "Inuit" translates to "people".

Tribalism implies the possession of a strong cultural or ethnic identity that separates one member of a group from the members of another group. Based on strong relations of proximity and kinship, as well as relations based on the mutual survival of both the individual members of the tribe and for the tribe itself, members of a tribe tend to possess a strong feeling of identity. Objectively, for a customary tribal society to form there needs to be ongoing customary organization, inquiry, and exchange. However, intense feelings of common identity can lead people to feel tribally connected.

The distinction between these two definitions of tribalism, objective and subjective, is an important one because while tribal societies have been pushed to the edges of the Western world, tribalism, by the second definition, is arguably undiminished. A few writers have postulated that the human brain is hard-wired towards tribalism, but that claim is usually linked to equating original questions of sociality with tribalism.