The voiced labio-velar approximant is a type of consonantal sound, used in certain spokenlanguages, including English. It is the sound denoted by the letter ⟨w⟩ in the English alphabet; likewise, the symbol in the International Phonetic Alphabet that represents this sound is ⟨w⟩, and the equivalent X-SAMPA symbol is w. In most languages it is a labialized velar approximant[ɰʷ], and the semivocalic counterpart of the close back rounded vowel[u] - i.e. the non-syllabic close back rounded vowel. As labio-velar consonants do not easily fit into consonant charts with only labial and velar columns, [w] may be put in either the velar column, (bi)labial column, or both, though putting it in both columns is rare; the placement may have more to do with phonological criteria than phonetic ones.
Features of the voiced labialized velar approximant:
Its manner of articulation is approximant, which means it is produced by narrowing the vocal tract at the place of articulation, but not enough to produce a turbulent airstream. The type of approximant is glide or semivowel. The term glide emphasizes the characteristic of movement (or 'glide') of /w/ from the /u/ vowel position to a following vowel position. The term semivowel emphasizes that, although the sound is vocalic in nature, it is not 'syllabic' (it does not form the nucleus of a syllable).
Martínez-Celdrán, Eugenio; Fernández-Planas, Ana Ma.; Carrera-Sabaté, Josefina (2003), "Castilian Spanish", Journal of the International Phonetic Association33 (2): 255–259, doi:10.1017/S0025100303001373