Air conditioning refers to the cooling and dehumidification of indoor air for thermal comfort. In a broader sense, the term can refer to any form of cooling, heating, ventilation or disinfection that modifies the condition of air. An air conditioner (often referred to as AC or air con.) is an appliance, system, or mechanism designed to stabilise the air temperature and humidity within an area (used for cooling as well as heating depending on the air properties at a given time), typically using a refrigeration cycle but sometimes using evaporation, most commonly for comfort cooling in buildings and motor-cars.
Air conditioning engineers broadly divide air conditioning applications into comfort and process.

Comfort applications aim to provide a building indoor environment that remains relatively constant in a range preferred by humans despite changes in external weather conditions or in internal heat loads.

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  Air conditioning makes deep plan buildings feasible, for otherwise they'd have to be built narrower or with light wells so that inner spaces receive sufficient outdoor air via natural ventilation. Air conditioning also allows buildings to be taller since wind speed increases significantly with altitude making natural ventilation impractical for very tall buildings. Comfort applications for various building types are quite different and may be categorized as:
  
• Low-Rise Residential buildings, including single family houses, duplexes, and small apartment buildings
• High-Rise Residential buildings, such as tall dormitories and apartment blocks
  Commercial buildings, which are built for commerce, including offices, malls, shopping centres, restaurants, etc.
• Institutional buildings, which includes hospitals, governmental, academic, and so on.
• Industrial spaces where thermal comfort of workers is desired.
• In addition to buildings, air conditioning can be used for many types of transportation - motor-cars and other land vehicles, trains, ships, aircraft, and spacecraft.

Process applications aim to provide a suitable environment for a process being carried out, regardless of internal heat and humidity loads and external weather conditions. Although often in the comfort range, it is the needs of the process that determine conditions, not human preference. Process applications include these:

Hospital operating theatres, in which air is filtered to high levels to reduce infection risk and the humidity controlled to limit patient dehydration. Although temperatures are often in the comfort range, some specialist procedures such as open heart surgery require low temperatures (about 18 °C, 64 °F) and others such as neonatal relatively high temperatures (about 28 °C, 82 °F).

Clean rooms for the production of integrated circuits, pharmaceuticals, and the like, in which very high levels of air cleanliness and control of temperature and humidity are required for the success of the process.

Facilities for breeding laboratory animals. Since many animals normally only reproduce in spring, holding them in rooms at which conditions mirror spring all year can cause them to reproduce year-round.

In both comfort and process applications the objective may be to not only control temperature, but also humidity, air quality and air movement from space to space.