Air handling unit from ANSI/AHRI Standard 430-2009 states that it is "A factory-made encased assembly consisting of a fan or fans and other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air.
The AHU is used to control the following parameters of the space
Temperature
Humidity
Air Movement
Air Cleanliness