The Airbase is a military structure, where Aircraft are built, upgraded and also operated from. The Airbase becomes available in the Industrial Age and, being a military structure, it can be built anywhere within friendly (own or allied) borders.
The Airbase can build three types of aircraft:
Fighter Aircraft (hotkey: F), effective against most units (especially against tanks,artillery) weak against buildings
Unlike most units, Bombers and Fighters can only operate from their assigned airbase and, like Missile Silos, Airbases also show a radius on the minimap, displaying the ranges that Fighters and Bombers may cover before returning to refuel. Its possible to recall selected aircraft (hotkey: Y) back to airbase. Aircraft can be also transferred from one airbase to another (hotkey: T). Repeat orders can be turned off (hotkey: E).