A VAV -- i.e., Variable Air Volume -- HVAC (heating, ventilation, and air conditioning) system that supplies air at varying levels or amounts at a constant temperature.
In contrast to constant airflow systems, which supply air at variable temperatures, VAV HVACs are preferred for their greater temperature control, longer parts life, reduced noise, and better energy efficiency.
An evolution of VAVs is multiple-zone VAV systems. These comprise of multiple VAV terminal units or boxes that diffuse air in specific areas of a large indoor space, such as an auditorium.
(Source: HPAC Engineering)