A nuclear family or elementary family is a family group consisting of a pair of adults and their children. This is in contrast to a single-parent family, to the larger extended family, and to a family with more than two parents. Nuclear families typically centre on a married couple; 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.