Messa di voce ['messa di 'vo:t?e] (Italian, "placing of voice") is a musical technique that involves a gradual crescendo and diminuendo while sustaining a single pitch. That is, a note is sung at a quiet volume, gradually and smoothly made louder until it reaches a high volume, then similarly made quiet again. The technique can be used on many instruments, but is perhaps best known for its use among singers.
Messa di voce should not be confused with mezza voce (Italian, "half voice") which means to sing at half strength.
The messa di voce is universally considered a very advanced vocal technique. To be properly executed, the only feature of the note being sung that should change is the volume - not the pitch, intonation, timbre, vibrato, and so on. This requires an extremely high level of vocal coordination, particularly in the diminuendo, so the technique is not often explicitly called for and is rarely heard outside classical music.
In Western art music, the messa di voce was associated with famous castrati such as Farinelli (and is now a mark of the mezzo-sopranos and countertenors who sing the same roles in Baroque operas). It was also popular in the bel canto period, when it was often used as a dramatic opening flourish for an aria, for example "Casta diva" from Norma. It became less common as the popular style of opera singing evolved from the light and elaborate music of that era to the louder and more speech-like singing of the middle and later 19th century.
In the popular music of the West messa di voce has been even less common. It still appears occasionally in some of the more ornate styles of popular music, however, like black gospel and other styles influenced by it. Modern mainstream vocalist with messa di voce include Beyoncé, Whitney Houston and Céline Dion.