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The hierarchy of biological classification's major eight levels. An order contains one or more families. Intermediate minor rankings are not shown.

In biological classification, family (Latin: familia, plural familiae) is a rank, or a taxon in that rank. Exact details of formal nomenclature depend on the Nomenclature Code which applies.

Example: "Walnuts and Hickories belong to the Walnut family" is a brief way of saying: the Walnuts (genus Juglans) and the Hickories (genus Carya) belong to the Walnut family (family Juglandaceae).

Family, as a rank intermediate between order and genus, is a relatively recent invention.

The term familia was coined by French botanist Pierre Magnol in his Prodromus historiae generalis plantarum, in quo familiae plantarum per tabulas disponuntur (1689) where he called families (familiae) the seventy-six groups of plants he recognised in his tables. The concept of rank at that time was still in statu nascendi, and in the preface to the Prodromus Magnol spoke of uniting his families into larger genera, which is far from how the term is used today.

Since the beginning of the 20th century, however, the term has been consistently used in its modern sense. Its usage and characteristic ending of the names belonging to this category are defined in the Codes of botanical and zoological nomenclature.

Almost all families are named for a type genus, adding the ending idae (animals) or aceae (plants) to the stem of the genus name. Exceptions are:

• Caprifoliaceae, Aquifoliaceae, and Fabaceae, named for their type species Lonicera caprifolium, Ilex aquifolia, and Vicia faba.
• Theaceae, named for Thea, a synonym of Camellia.
• Eight families of plants with alternate names. Fabaceae is also called Leguminosae, Poaceae Gramineae, etc.
• Elapidae. The type genus is Homoroselaps, which was originally named Elaps but was temporarily moved to a different family and the name changed as a result.

• Systematics - the study of the diversity of life
• Cladistics - the classification of organisms by their order of branching in an evolutionary tree
• List of families - list of family names in the taxonomy of biology
• Phylogenetics - the study of evolutionary relatedness among various groups of organisms
• Taxonomy
• Compare Family
• Scientific classification
• Rank (botany)
• Rank (zoology)
• Virus classification
• protein family
• gene family