In linguistics, areal features are elements shared by languages or dialects in a geographic area, particularly when such features are not descended from a proto-language, or, common ancestor language. That is, an areal feature is contrasted to genealogically determined similarity within the same language family. Features may diffuse from one dominant language to neighbouring languages (see "sprachbund").
Resemblances between two or more languages (whether in typology or in vocabulary) can be due to genetic relation (descent from a common ancestor language), to borrowing between languages, to retention of features when a population adopts a new language, or simply by coincidence. When little or no direct documentation of ancestor languages is available, determining whether the similarity is genetic or areal can be difficult. Edward Sapir notably used evidence of contact and diffusion as a negative tool for genetic reconstruction, treating it as a subject in its own right only at the end of his career (e.g., for the influence of Tibetan on Tocharian).
Genetic relationships are represented in the family tree model of language change, and areal relationships are represented in the wave model. William Labov in 2007 reconciled these models in a general framework based on differences between children and adults in their language learning ability. Adults do not preserve structural features with sufficient regularity to establish a norm in their community, but children do. Linguistic features are diffused across an area by contacts among adults. Languages branch into dialects and thence into related languages through small changes in the course of children's learning processes which accumulate over generations, and when speech communities do not communicate (frequently) with each other, these cumulative changes diverge. Diffusion of areal features for the most part hinges on low-level phonetic shifts, whereas tree-model transmission includes in addition structural factors such as "grammatical conditioning, word boundaries, and the systemic relations that drive chain shifting".
In some areas with high linguistic diversity, a number of areal features have spread across a set of languages to form a sprachbund (also known as a linguistic area, convergence area or diffusion area). Some examples are the Balkan sprachbund, the Mainland Southeast Asia linguistic area, and the languages of the Indian subcontinent.