originally, a contact variety of the Greek language that was spoken throughout the eastern Mediterranean region during the Hellenic and Roman empires. The term comes from the Greek koine (“common” or “shared”), although the variety was based chiefly on the Attic Greek (Attic dialect) dialect. A compromise variety, this original koine consisted of features easily recognizable to speakers of most Greek dialects (dialect) and dispensed with those that most often impeded mutual intelligibility. In linguistics, the term koine is now applied to any modified language variety that has developed from contact between dialects of the same language or, in some cases, between languages that are genetically or typologically related.

Unlike creoles (creole languages) and pidgins (pidgin), koines are considered to be genetically related to the language varieties from which they have evolved. That is, they remain dialects of the primary languages to which they are related grammatically and lexically (in terms of vocabulary). Since no genetically unrelated languages were involved in the contacts that produced them, the structures of koines are not as drastically divergent from those of their ancestor languages as those of creoles and pidgins.

Koines may be written or spoken. Historical examples of koines include Standard Macedonian, the Italian of late 14th-century Naples, and the language of northern China in the 7th–10th centuries. It is assumed that koines also evolved in the earliest British colonies in North America, Australia, and New Zealand in response to the different metropolitan dialects that the colonists brought with them. Analogous varieties must have evolved in colonial Spanish, Portuguese, French, and Dutch settlements, which accounts partly for why the colonial non-creole varieties of the relevant European languages diverged from their metropolitan counterparts. In cases where a koine and a creole have evolved out of the same language (as with early colonial English and Gullah), the koine (rather than its metropolitan ancestor dialects or languages) is assumed to have been the starting point for both. Some argue that the formation of colonial-era koines started in European port cities where speakers of related dialects met before emigrating to the colonies.