A nuclear family is a household consisting of two married, heterosexual parents and their legal children (siblings), as distinct from the extended family. Whilst the family is a near-universal cultural phenomenon, nuclear families do not form the family unit in every society. Nuclear families are typical in societies where people must be relatively mobile — such as hunter-gatherers and industrial societies. Although, as time progresses, the ideal family image is slowly shifting from the aforementioned to something like that of an amiably divorced couple with joint custody of their children.
1. http://www.hewett.norfolk.sch.uk/CURRIC/soc/family/fam2.htm (authorized users only)