The definition of a coil in HVAC (Heating, Ventilation, and Air Conditioning) is a device used to transfer heat from one medium to another. Coils are typically made of metal and are used to heat or cool air, depending on the application. In a heating system, the coil absorbs heat from the air and transfers it to the air inside the building. In an air conditioning system, the coil absorbs heat from the air inside the building and transfers it to the outside air. Coils are an important part of any HVAC system and can be found in both residential and commercial buildings. They are also used in industrial applications such as in the production of food and beverages. The efficiency of a coil is determined by its size, material, and the amount of air that it can move. Coils can be made of copper, aluminum, steel, or other materials, and they come in a variety of shapes and sizes. Proper maintenance and installation of coils is essential to ensure the efficiency and longevity of the system.