In biological classification, family (familia, plural familiae) is one of the eight major ; it is classified between order and genus. A family may be divided into subfamily, which are intermediate ranks above the rank of genus. In common name, a family may be named after one of its common members; for example, and hickory trees belong to the family Juglandaceae, commonly known as the walnut family.
What does or does not belong to a family—or whether a described family should be recognized at all—are proposed and determined by practicing taxonomists. There are no hard rules for describing or recognizing a family, or any taxa. Taxonomists often take different positions about descriptions of taxa, and there may be no broad consensus across the scientific community for some time. Some described taxa are accepted broadly and quickly, but others only rarely, if at all; the publishing of new data and opinion often enables adjustments and consensus over time.
Carl Linnaeus used the word familia in his Philosophia botanica (1751) to denote major groups of plants: , , , Palmae, and so on. 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 (or ordo naturalis). In nineteenth-century works such as the Prodromus of Augustin Pyramus de Candolle and the Genera Plantarum of George Bentham and Joseph Dalton Hooker this word ordo was used for what now is given the rank of family.
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 (some of them were not named) in some but not in all his orders of "insects" (which then included all ).