Airspace refers to the region of the atmosphere and outer space, controlled by a particular country or organization, within which air traffic control authority is exercised. It includes all altitudes above the territory of a given state, as well as that state's territorial waters. Airspace can be classified into different categories based on factors such as altitude, use, and level of restrictions. These categories include Class A, B, C, D, and E airspaces, each with its own set of rules and requirements for aircraft operations within them.