A nuclear family, elementary family, cereal-packet family or conjugal family is a family group consisting of parents and their children (one or more), typically living in one home residence. It is in contrast to a single-parent family, the larger extended family, or a family with more than two parents. Nuclear families typically center on a heterosexual married couple which may have any number of children. There are differences in definition among observers. Some definitions allow only biological children who are full-blood siblings and consider adopted or half and step siblings a part of the immediate family, but others allow for a step-parent and any mix of dependent children, including stepchildren and adopted children. Most sociologists and anthropologists consider the nuclear family as the most basic form of social organization, while others consider the extended family structure to be the most common family structure in most cultures and at most times.

The term nuclear family was popularized in the 20th century. In the United States, it became the most common form of family structure in the 1950s, 1960s and 1970s. Since that time, the number of North American nuclear families is gradually decreasing, while the number of alternative family formations has increased; this phenomenon is generally opposed by members of such philosophies as social conservatism or familialism, which consider the nuclear family structure important.