labialisation

n. modification of pronunciation of words by contraction of the lip opening, pronouncing by rounding the lips (also labialization)

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.
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n. labialization