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Labialisation is a secondary articulatory feature of sounds in some languages. Labialized sounds involve the lips while the remainder of the oral cavity produces another sound. The term is normally used to refer to consonants. When vowels involve the lips, they are usually called rounded.

Labialisation may also refer to a type of assimilation process.

Labialisation is the most widespread secondary articulation in the world's languages. It is phonemically contrastive in the Northwest Caucasian, Athabaskan, Salishan, and Indo-European language families, among others.

American English has three degrees of (phonetic) labialization: Fully rounded /w/ and initial /ɹ/, open-rounded /ʃ/, /ʒ/, /ʧ/, /ʤ/, and unrounded, which in vowels is sometimes called spread. These secondary articulations are not universal. For example, while French shares the English open-rounding of /ʃ/, /ʒ/, Russian does not. Such distinctions are helpful for non-native English speakers whose native languages use different articulations for sounds such as "r" and "l".

The most common form of labialisation is rounding of dorsal consonants such as k, g, and q. With non-dorsal consonants, labialisation prototypically involves velarization as well, so it might more accurately be called labiovelarisation. However, this is not always the case, and labialisation is not restricted to lip-rounding. The following articulations have either been described as labialisation, or been found as allophonic realisations of prototypical labialisation:

• Labial rounding, with or without protrusion of the lips (found in Navajo)^{[citation needed]}
• Labiodental frication, found in Abkhaz^{[citation needed]}
• Bilabial frication, found in Ubykh^{[citation needed]}
• Bilabial trill, found in Ubykh^{[citation needed]}
• Bilabial plosion, found in Ubykh^{[citation needed]}
• "Labilialisation" without lip rounding, found in the Iroquois
• Rounding without velarization, found in Shona and in the Bzyb dialect of Abkhaz

Eastern Arrernte is a language with labialisation at all places and manners of articulation. The labialisation derives historically from adjacent rounded vowels, as is also the case of the Northwest Caucasian languages.

In the International Phonetic Alphabet, labio-velarization of consonants is indicated with a raised w modifier [ʷ] (Unicode U+02B7), as in /kʷ/. There are also diacritics, respectively [ɔ̹], [ɔ̜], to indicate greater or lesser degrees of rounding. These are normally used with vowels, but may occur with consonants. For example, in the Athabaskan language Hupa, voiceless velar fricatives distinguish three degrees of labialization, transcribed either /x/, /x̹/, /xʷ/ or /x/, /x̜ʷ/, /xʷ/.

The Extended IPA has two additional symbols for degrees of rounding: Spread /ɹ͍/ and open-rounded /ʒ^œ/. It also has a symbol for labialdentalized sounds, /t^ʋ/.

If precision is desired, the Abkhaz and Ubykh articulations may be transcribed with the appropriate fricative or trill raised as a diacritic: [t^v], [t^β], [t^ʙ], [t^p].

For simple labialization, Ladefoged and Maddieson resurrected an old IPA symbol, [ ̫]. In Shona, /s̫/ and /z̫/ contrast with /s/ and /z/, and in some dialects with /sw/, /zw/, /s̫w/, /z̫w/] as well. The open rounding of English /ʒ^œ/ is also simple (unvelarized).

Labialisation also refers to a specific type of assimilatory process where a given sound become labialised due to the influence of neighboring labial sounds. For example, /k/ may become /kʷ/ in the environment of /o/, or /a/ may become /o/ in the environment of /p/ or /kʷ/.

In the Northwest Caucasian languages as well as some Australian languages rounding has shifted from the vowels to the consonants, producing a wide range of labialized consonants and leaving in some cases only two phonemic vowels. This appears to have been the case in Ubykh and Eastern Arrernte, for example. The labial vowel sounds usually still remain, but only as allophones next to the now-labial consonant sounds.

• Peter Ladefoged & Ian Maddieson, The Sounds of the World's Languages (1996) ISBN 0-631-19815-6