A clan is a group of people united by kinship and descent, which is defined by actual or perceived descent from a common ancestor. Even if actual lineage patterns are unknown, clan members may nonetheless recognize a founding member or apical ancestor. The kinship-based bonds may be merely symbolical in nature, whereby the clan shares a "stipulated" common ancestor which is a symbol of the clan's unity. When this ancestor is not human, this is referred to as animallian totem. Clans can be most easily described as tribes or sub-groups of tribes. The word clan is derived from 'clann' meaning 'children' in the Gaelic languages Irish and Scottish Gaelic language and the word entered into English as a label for the tribal nature of Irish and Scottish Gaelic society in about the year 1425. The Gaelic term for clan is fine /finɨ/. In the 20th century, the word entered into use internationally in scientific descriptions of territorial and group behavior in various species of mammals, including hyenas, badgers and rats.

Organization of clans in anthropology

Some clans are patrilineal, meaning its members are related through the male line; for example, the clans of Armenia. Others are matrilineal; its members are related through the female line. Still other clans are bilateral, consisting of all the descendants of the apical ancestor through both the male and female lines; the clans of Scotland are one example. Whether a clan is patrilineal, matrilineal, or bilateral depends on the kinship rules and norms of their society.

In different cultures and situations a clan may mean the same thing as other kin-based groups such as tribes and bands. Often, the distinguishing factor is that a clan is a smaller part of a larger society such as a tribe, a chiefdom, or a state. Examples include Scottish, Irish, Chinese, Japanese clans and Rajput clans in India and Pakistan, which exist as kin groups within their respective nations. Note, however, that tribes and bands can also be components of larger societies. Arab tribes are small groups within Arab society, and Ojibwa bands are smaller parts of the Ojibwa tribe. In some cases multiple tribes recognized the same clans, such as the bear and fox clans of the Chickasaw and Choctaw tribes.

Apart from these different traditions of kinship, further conceptual confusion arises from colloquial usages of the term. In post-Soviet countries for example it is quite common to speak of clans referring to informal networks within the economic and political sphere. This usage reflects the assumption that their members act towards each other in a particularly close and mutually supportive way approximating the solidarity among kinsmen. However, the Norse clans, the ätter, can not be translated with tribe or band, and consequently they are often translated with house or line.

Polish clans differ from most others in being a collection of families bearing the same coat of arms, as opposed to actually claiming a common descent. This is discussed under the topic of Polish Heraldry.