Labialisation is a secondary articulatory feature of phonemes in a language, most usually used to refer to consonants. Labialisation, simply put, is the usage of the lips as a secondary articulator while the remainder of the oral cavity produces some other phoneme.
While labialisation is by no means universal in the world's languages, it is certainly extremely widespread. It appears in families as varied as Northwest Caucasian, Athabaskan, Salishan and (from a diachronic perspective) Indo-European, where it is thought that the proto-language, Proto-Indo-European, used labialised velar consonants, and MinoanGreek appears to have used it as well.
Labialisation is not restricted to lip-rounding, although this is certainly the most common type. The following labial articulations have been found as realisations of labialisation:
Labial rounding, with or without protrusion of the lips (found in Navajo)
For example, the voicelesslabializedvelarplosive [kʷ] has only a single stop articulation, velar [k], with a simultaneous [w]-like rounding of the lips, and is usually heard as a kind of [k].
The most frequently encountered are labialization (such as [kʷ]), palatalization (such as the Russian "soft" consonant [tʲ]), velarization (such as the English "dark" L [lˠ]), and pharyngealization (such as the Arabic "emphatic" consonant [tˤ]).
For this reason, the IPA symbols for labialization and palatalization were for a time placed directly under the consonant (as [k̫] and [ƫ]), and there is still an alternate symbol for velarization or pharyngealizaton that is superposed across the consonant (as in [ɫ] for dark L).
Labialized sounds involve the lips while the remainder of the oral cavity produces another sound.
American English has three degrees of labialization: Fully rounded /w/ and initial /ɹ/, open-rounded /ʃ ʒ t͡ʃ d͡ʒ/, and unrounded, which in vowels is sometimes called spread.
The most common form of labialization is rounding of dorsal consonants such as k, g, and q.