- 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).
Next only to species and genus, the family is the most important rank in taxonomy. Exact details of formal nomenclature depend on the Nomenclature Code which applies, see scientific classification and:
History of the concept
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.
Carolus Linnaeus used the word familia in his Philosophia botanica (1751) to denote major groups of plants; trees, herbs, ferns, palms, etc. He used this term only in the morphological section of the book, discussing the vegetative and generative organs of plants. Subsequently, in French botanical publications, from Michel Adanson's Familles naturelles des plantes (1763) and until the end of the 19th century, the word famille was used as a French equivalent of the Latin ordo. It should be noted that the word ordo in nineteenth century works such as the Prodromus of de Candolle and the Genera Plantarum of Bentham & Hooker was used for what now is given the rank of family (see ordo naturalis).
In zoology, the family as a rank intermediate between order and genus was introduced by Pierre André Latreille in his Précis des caractères génériques des insectes, disposés dans un ordre naturel (1796). He used families (part of them not named) in some but not in all his orders of "insects" (which then included all arthropods).
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.