Family (Latin: familia, pl.: familiae) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between order and genus. A family may be divided into subfamilies, which are intermediate ranks between the ranks of family and genus. The official family names are Latin in origin; however, popular names are often used: for example, walnut trees and hickory trees belong to the family Juglandaceae, but that family is commonly referred to as the "walnut family".

The delineation of what constitutes a family—or whether a described family should be acknowledged—is established and decided upon by active taxonomists. There are not strict regulations for outlining or acknowledging a family, yet in the realm of plants, these classifications often rely on both the vegetative and reproductive characteristics of plant species. Taxonomists frequently hold varying perspectives on these descriptions, leading to a lack of widespread consensus within the scientific community for extended periods.

The naming of families is codified by various international bodies using the following suffixes:

Name changes at the family level are regulated by the codes of nomenclature. For botanical families, some traditional names like Palmae (Arecaceae), Cruciferae (Brassicaceae), and Leguminosae (Fabaceae) are conserved alongside their standardized -aceae forms due to their historical significance and widespread use in the literature. Family names are typically formed from the stem of a type genus within the family. In zoology, when a valid family name is based on a genus that is later found to be a junior synonym, the family name may be maintained for stability if it was established before 1960. In botany, some family names that were found to be junior synonyms have been conserved due to their widespread use in the scientific literature.

The family-group in zoological nomenclature includes several ranks: superfamily (-oidea), family (-idae), subfamily (-inae), and tribe (-ini). Under the principle of coordination, a name established at any of these ranks can be moved to another rank while retaining its original authorship and date, requiring only a change in suffix to reflect its new rank.

New family descriptions are relatively rare in taxonomy, occurring in fewer than one in a hundred taxonomic publications. Such descriptions typically result from either the discovery of organisms with unique combinations of characters that do not fit existing families, or from phylogenetic analyses that reveal the need for reclassification.

The taxonomic term familia was first used by French botanist Pierre Magnol in his Prodromus historiae generalis plantarum, in quo familiae plantarum per tabulas disponuntur (1689) where he called the seventy-six groups of plants he recognised in his tables families (familiae). The concept of rank at that time was not yet settled, 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.

In his work Philosophia Botanica published in 1751, Carl Linnaeus employed the term familia to categorize significant plant groups such as trees, herbs, ferns, palms, and so on. Notably, he restricted the use of this term solely within the book's morphological section, where he delved into discussions regarding the vegetative and generative aspects 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).