Code-switching may simply be defined as the use of at least two languages within the same discourse. Discourse, here, refers to any instance of language use for communicative purposes. So if a bilingual (or multilingual), in its broad sense, starts a sentence in English, and the response to his utterance from another speaker is in Turkish, this may be called code-switching, but only if the ongoing communication process is mutually intelligible to both or all of the speakers. Code-switching may occur inter-sententially or intra-sententially. If the latter is considered, the phenomenon is called code-mixing. In other words, if the switch is within sentences or word boundaries, it is code-mixing. Contrary to this, if the switch is across sentence boundaries, the phenomenon under discussion is code-switching. To illustrate, the examples below can be given:
Jeff: Ahmet, Is this your kitap? (English-Turkish code-switching)
Ahmet: Hayır. But that is my defter.
Jeff: Ahmet, Is this your book?
Ahmet : No. But that is my notebook.
Considering the sentences given above, example 1 is an instance of code-mixing appearing in the form of intra-sentential switching. When example 2 is considered, it is obvious that code-switching across sentence boundaries exists, as complete sentences uttered by the interlocutors follow each other, including switches between English and Turkish. It is only natural to suppose that in order to perform an alternation between languages in speaking or writing, the participants should be competent in the languages being used. Throughout the history of bilingual research, many researchers have studied the code-switching phenomenon from different perspectives, at different levels, and in a great array of domains and settings.
Among these, language production in bilinguals (either early or late), language contact, immigrant groups, minorities, bi/multilingual communities, education and foreign language teaching are the leading fields which have attracted researchers. It has been the case that psycholinguistic, sociolinguistic, neurolinguistic and pedagogy-driven perspectives have handled code-switching phenomenon with a variety of motives and research designs, each of which should be recognized in its own methodological framework.