Merriam-Webster dates the term back to 1947, whilst the Oxford English Dictionary has a reference to the term from 1925; thus it is relatively new, although nuclear family structures themselves date back thousands of years. The term nuclear is used in its general meaning referring to a central entity or "nucleus" around which others collect.
In its most common usage, the term nuclear family refers to a household consisting of a father, a mother and their children all in one household dwelling. George Murdock, an early and influential observer of families, describes the term in this way:
The family is a social group characterized by common residence, economic cooperation and reproduction. It contains adults of both sexes, at least two of whom maintain a socially approved sexual relationship, and one or more children, own or adopted, of the sexually cohabiting adults.
The nuclear family or elementary family is a term used to define a family group consisting of a pair of adults and their children. This is in contrast to a polygamous family, single-parent family, and to the larger extended family. Nuclear families typically center on a married couple, but not always; the nuclear family may have any number of children. There are differences in definition among observers; some definitions allow only biological children that are full-blood siblings, while others allow for a stepparent and any mix of dependent children including stepchildren and adopted children.
Family structures of a single married couple and their children were present in Western Europe and New England in the 17th century, influenced by church and theocratic governments. With the emergence of proto-industrialization and early capitalism, the nuclear family became a financially viable social unit. The term nuclear family first appeared in the early twentieth century. Alternative definitions have evolved to include family units headed by same-sex parents, and perhaps additional adult relatives who take on a cohabiting parental role; in this later case it also receives the name of conjugal family.
The concept that a narrowly defined nuclear family is central to stability in modern society has been promoted by modern social conservatives in the United States, and has been challenged as historically and sociologically inadequate to describe the complexity of actual family relations.