Language contact occurs when speakers of two or more languages or varieties interact and influence each other. The study of language contact is called contact linguistics.
Multilingualism has likely been common throughout much of human history, and today most people in the world are multilingual.
When speakers of different languages interact closely, it is typical for their languages to influence each other. Language contact can occur at language borders, between adstratum languages, or as the result of migration, with an intrusive language acting as either a superstratum or a substratum.
Language contact occurs in a variety of phenomena, including language convergence, borrowing and relexification. The most common products are pidgins, creoles, code-switching, and mixed languages. Other hybrid languages, such as English, do not strictly fit into any of these categories.
The most common way that languages influence each other is the exchange of words. Much is made about the contemporary borrowing of English words into other languages, but this phenomenon is not new, nor is it very large by historical standards. The large-scale importation of words from Latin, French and other languages into English in the 16th and 17th centuries was more significant. Some languages have borrowed so much that they have become scarcely recognizable. Armenian borrowed so many words from Iranian languages, for example, that it was at first considered a branch of the Indo-Iranian languages. It was not recognized as an independent branch of the Indo-European languages for many decades.