Tanglish refers to the macaronic code-mixing or code-switching of the Tamil and English languages, in the context of colloquial spoken language. In the context of colloquial written language, Tanglish refers to the transliteration of Tamil text in English alphabet (Roman Tamil), with extensive usage of English vocabulary.

The name is a portmanteau of Tamil and English, and has taken various forms over time. The earliest form is Tamilish (dating from 1972), then Tinglish (1974), Tamglish (1991), Tash (1993), Thanglish (1997), and Tanglish (1999).

Tanglish has become the de-facto style of Tamil spoken in urban areas of present-day Tamil Nadu as well as contemporary Tamil cinema, to the extent that even words that have native colloquial equivalents are often replaced with English words.

In modern day India, since English is perceived as the prestige language over Indian languages by common people, Tanglish subsequently is perceived as the high prestige variety (H) of spoken Tamil, while vernacular colloquial Tamil (without English influence) is considered low variety (L) of spoken Tamil.

A study of code switching in everyday speech in Tamil Nadu found that English words are commonly inserted into sentences that otherwise follow Tamil syntax.

A characteristic of Tanglish or Tamil-English code-switching is the addition of Tamil affixes to English words. The sound "u" is added at the end of an English noun to create a Tamil noun form, as in "sound-u" and the words "girl-u heart-u black-u" in the lyrics of "Why This Kolaveri Di". English nouns often are combined with Tamil case markers, as in "journey-ai" (accusative case), "driver-kku" (dative case, used to mean "for the driver"), and "teacher-oḍa" (of the teacher, genitive case). Verbs and some nouns from the English language are converted to Tamil verb forms by adding Tamil verbalizers that indicate verb mood. For example, the Tamil verb "paṇṇu" (imperative mood "do") is added to the English verb "drive", resulting in "drive paṇṇu", used to mean "do the driving". Another pattern that has been noted by speakers or observers of Tanglish is the addition of the syllable "-fy" at the end of a Tamil word (e.g., maatti-fy, Kalaachi-fy).

The below examples show how code-mixing is done with English in Spoken Tamil. In terms of vocabulary, the major differences between formal, colloquial and anglicised Tamil is that, formal Tamil tends to draw most of its vocabulary from Pure Tamil (செந்தமிழ், inspired from Old Tamil), colloquial Tamil (கொடுந்தமிழ்) has significant number of loanwords from Prakrit & Sanskrit, and Tanglish replaces a large amount of nouns and verbs with English words.

The use of Tanglish is common in Chennai, possibly due in part to the use of English in education. The influx of speakers of other languages (such as Telugu, Gujarati, and Kannada) to the city has also increased the importance of English as the lingua franca. In The Hindu in 2010, a student in Chennai told of the widespread use of Tanglish by teenagers in her city. She said Tanglish was "something almost every teenager in Chennai uses", but noted that her mother said Tanglish was "murdering the [Tamil] language". That same year, a Tamil teacher in a matriculation school in Chennai reported that few of her students had a large enough Tamil vocabulary to be able to speak Tamil without including some words of English.