1. a kinship unit consisting of a group of individuals united by blood or by marital, adoptive, or other intimate ties. Although the family is the fundamental social unit of most human societies, its form and structure vary widely. See biological family; extended family; nuclear family; permeable family; stepfamily.

2. in biological taxonomy, a main subdivision of an order, consisting of a group of similar, related genera (see genus).

3. a collection of mathematically or statistically related entities. For example, a set of statistical tests conducted when there are more than two groups for an independent variable within an analysis of variance constitutes a family of tests. See also family-wise alpha level; family-wise error rate. —familial adj.