What is "messa di voce"

Messa di voce (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.