The Collaborative International Dictionary
Labialization \La`bi*al*i*za"tion\, n. (Phonetics) The modification of an articulation by contraction of the lip opening.
Wiktionary
n. (context linguistics English) A secondary articulatory feature of usually consonants that involves the contraction or rounding of the labia during pronunciation.
Wikipedia
Labialization 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 restricted to consonants. When vowels involve the lips, they are called rounded.
The most common labialized consonants are labialized velars. Most other labialized sounds also have simultaneous velarization, and the process may then be more precisely called labio-velarization.
Labialization may also refer to a type of assimilation process.