An acronym meaning "Heating, Ventilation, Air-Conditioning", thus covering almost all of the things that can be done to the ambient air within a building to render it fresh and of the desired temperature.
The operation of HVAC is based on scientific principles of thermodynamics, fluid mechanics, and heat transfer.
An advanced HVAC system is computer controlled and will include temperature sensors, humidity sensors, and Carbon Dioxide sensors. It will incorporate a timer system to conserve energy when the building is unoccupied.
HVAC design is a specialised branch of engineering.