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' reflects 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.
Estimates of the number of living languages vary from 5,000 to 8,000, depending on the precision of one's definition of "language", and in particular on how one classifies dialects. The 2013 edition of Ethnologue catalogs just over 7,000 living human languages. A "living language" is simply one that is used as the primary form of communication of a group of people. There are also many dead and extinct languages, as well as some that are still insufficiently studied to be classified, or are even unknown outside their respective speech communities.
Membership of languages in a language family is established by comparative linguistics. Sister languages are said to have a "genetic" or "genealogical" relationship. The latter term is older, but has been revived in recent years to better distinguish the relationships between languages from the genetic relationships between people. The evidence of linguistic relationship is found in observable shared characteristics that are not attributed to contact or 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.