In today's world, all different types of family units exist including half-siblings, stepparents, adopted children and different combinations of all of the above. However, we still use the terms "nuclear family" and "extended family" to define two different types of family groups.
A Nuclear Family
A nuclear family, named from the idea of it being the family at the "nucleus" of other family groups, consists of a father, mother and any number of their children. It is defined by blood relation of the parents and children and the marriage of the parents. Because this term does not include other variations of family units, it is becoming antiquated as it is more common to see families that do not fit into its constructs.
The Extended Family
The extended family is family is usually define by some common genetic lineage that falls outside of the nuclear family. It consists of aunts, uncles, cousins and grandparents. It can be traced back for many generations and can be quite large. In the United States, it is less common for extended families to share a home as it is in some other countries.
Families Around the World
The general description of "nuclear" and "extended" families was created around the early 1900s. In other parts of the world, the definition of "nuclear" family may be different and include aunts, uncles and cousins that wouldn't be defined as "nuclear" or "immediate" families in most Western cultures. Societies in Africa, Asia, Latin America and many other parts of the world include family other than parents and siblings as their "immediate" family.
Problems with the definition of nuclear family
The term "nuclear family" does not adequately cover what most people consider family. Today, there is a diverse range of family groups. Marriage, divorce, adoption, same-sex couples and single-parent homes make up a large number of family units are not represented within the description of "nuclear" family.