Definitions of "family" vary tremendously over period, region, and cultural traditions. Even sociologists vary in their definition of family, with many sociologists and anthropologists emphasizing that what is considered a family depends on particular cultural factors and thus can only be considered relative to a specific society.
In most developed societies, the term "family" has a specific legal meaning. For example, in ancient Rome, a family consisted of the oldest man in the household (the pater familias), his wife, dependent unmarried female relatives, unmarried female children, unemancipated male children, slaves, and freedmen. These were all under patria potestas (the power of the father) which included financial control and even, in many cases, the right to inflict capital punishment (a father could legally kill his slaves or children).
In early modern Europe, extended families, including a couple, elderly relatives, unmarried female relatives, and children, often would live together as a family unit in a single household, while in the twentieth century, the concept of a nuclear family of a man, woman, and children became more common. Some cultures, include the Arabic one and the Mormons, traditionally practiced polygamy, in which one man had multiple wives and all lived together as a single family unit.
The late twentieth century has ushered in more diverse conceptions of family, included blended families (with divorced couples remarrying), gay couples (sometimes with children, either biological or adopted), and other relationships defined by personal affinities rather than kinship.
Sociologists also disagree on how the family might function in society as to whether it is primarily an economic unit, one that increases social cohesion, one that legitimizes and structures personal affinities, or one which seeks to reinforce class structures.