A vocalization was a sound made by a lifeform to communicate. Examples included barking (dogs), bird calls (birds), hissing, purring, and meowing (cats), and roaring (Klingon females).
Data was able to distinguish between three hundred different vocalizations. In 2370, he mentioned this when Jean-Luc Picard heard what sounded like an animal calls on the USS Enterprise-D. Picard noted that Worf, while suffering from Barclay's Protomorphosis Syndrome, was making angry, aggressive sounds, and speculated that the apparently devolved Klingon might be attempting to scare off other predators, mark his territory, or commence the mating process. (TNG: "Genesis")