The list of units
Aircraft Carriers carry air units and are legitimate airbases. Air units may re-base to carriers, and can launch missions from them. Carriers are equipped with Radar, so they can see two squares in all directions, regardless of any blocking terrain.
A coastal city needs oil in its Strategic Resource box to build a carrier.
The aircraft carrier first came into widespread use during World War II. Carriers act as floating airfields, carrying fighters and bombers far from friendly territory to be launched against enemy targets. This allows an air strike against targets on both land and sea that would otherwise be far out of range. The range of the aircraft launched from carriers forever changed the methodology of naval battles. Opposing fleets could now engage in battle without either fleet ever entering the other's visual range, ending the dominance of the battleship as the strongest vessel on the sea.