A language family is a group of languages related through descent from a common ancestor, called the proto-language of that family. The term 'family' comes from the tree model of language origination in historical linguistics, which makes use of a metaphor comparing languages to people in a biological family tree, or in a subsequent modification, to species in a phylogenetic tree of evolutionary taxonomy. No actual biological relationship between speakers is implied by the metaphor.
As of early 2009, SIL Ethnologue catalogued 6,909 living human languages. A "living language" is simply one that is widely used as a primary form of communication by a specific group of living people. The exact number of known living languages varies from 5,000 to 10,000, depending generally on the precision of one's definition of "language", and in particular on how one classifies dialects. There are also many dead and extinct languages.
Membership of languages in the same language family is established by comparative linguistics. Daughter languages are said to have a genetic or genealogical relationship; the former term is more modern, while the latter is more traditional. The evidence of linguistic relationship is found in observable shared characteristics that are not attributed to borrowing. Genealogically related languages present shared retentions, that is, features of the proto-language (or reflexes of such features) that cannot be explained by chance or borrowing (convergence). Membership in a branch or group within a language family is established by shared innovations, that is, common features of those languages that are not found in the common ancestor of the entire family. For example, Germanic languages are "Germanic" in that they share vocabulary and grammatical features that are not believed to have been present in the Proto-Indo-European language. These features are believed to be innovations that took place in Proto-Germanic, a descendant of Proto-Indo-European that was the source of all Germanic languages.
A family is a monogenetic unit; that is, all its members derive from a common ancestor, and all attested descendants of that ancestor are included in the family. However, unlike the case of biological nomenclature, every level of language relationship is commonly called a family. For example, the Celtic, the Germanic, Slavic, Romance, and Indo-Iranian language families are branches of a larger Indo-European language family.
Language families can be divided into smaller phylogenetic units, conventionally referred to as branches of the family because the history of a language family is often represented as a tree diagram. However, the term family is not restricted to any one level of this "tree". The Germanic family, for example, is a branch of the Indo-European family. (In this way, the term family is analogous to the biological term clade.) Some taxonomists restrict the term family to a certain level, but there is little consensus in how to do so. Those who affix such labels also subdivide branches into groups, and groups into complexes. A top-level (largest) family is often called a phylum or stock. The term superfamily is sometimes applied to proposed groupings of language families whose status as phylogenetic units is generally considered to be unsubstantiated by accepted historical linguistic methods.