This term specifically refers to the singing of vocalises. It can also take on a more general meaning of singing to vowel sounds or humming (singing with closed lips). As with the vocalise, vocalization is most often done as an exercise for the voice, however, there are solo and choral compositions that employ vocalization.
Occasionally, the melismas of Gregorian chant or the coloratura of 18th-century opera are referred to as vocalization, but this is not accurate. Melisma and coloratura are very different in both purpose and sound.
A similar technique is also used in jazz music and is known as Vocalese.
See also Vocalise; Vocalese.