The air starts from the lungs, passes through the trachea, moves (or does not move) the vocal cords of the larynx and through the pharynx is channeled into the oral cavity (less than the nasal cavity) where it is formed acoustically by the shapes, positions and the movements of the “vocal organs” (tongue, lips, soft palate, lower jaw), in relation to the “reference system” (teeth, gums, hard palate, upper jaw).
Babiniotis (2006)

Functional definition

An acoustically “pleasant” result of a subtle and well-timed movement of a person’s articulation and speech structures.