An ambiguous term, referring to both the group formed by a co-resident husband, wife, and children (which sociologists term the nuclear family) or to a wider category of relatives, including non-resident grandparents, uncles, aunts, cousins, etc (the extended family). The nuclear family was once regarded as the key domestic institution of modern Western societies, but marriage has become somewhat less common, and the divorce rate has greatly increased, so that in societies such as contemporary England and the USA the majority of the population no longer lives within a nuclear family group.

According to some estimates, only c.20% of all households are made up of nuclear families, the rest being constituted by single parents, foster, childless, or extended families, or simply individuals living alone. For different reasons, the same may have been true in many European peasant communities and in the early industrial cities.

In many parts of the world, and in Europe in the pre-industrial period, the nuclear family was commonly part of a larger domestic group including some other relatives, and also employees, apprentices, etc. Anthropologists have been particularly interested in the circle of kin beyond the nuclear family, and have demonstrated that kinship groupings wider than the nuclear family may have crucial social functions.