Airspace means the portion of the atmosphere controlled by a country above its territory, including its territorial waters or, more generally, any specific three-dimensional portion of the atmosphere.
Airspace may be further subdivided into a variety of areas and zones, including those where there are either restrictions on flying activities or complete prohibition of flying activities.
By international law, the notion of a country's sovereign airspace corresponds with the maritime definition of territorial waters as being 12 nautical miles (22.2 km) out from a nation's coastline. Airspace not within any country's territorial limit is considered international, analogous to the "high seas" in maritime law. However, a country may, by international agreement, assume responsibility for controlling parts of international airspace, such as those over the oceans. For instance, the United States provides air traffic control services over a large part of the Pacific Ocean, even though the airspace is international.
There is no international agreement on the vertical extent of sovereign airspace (the boundary between outer space— which is not subject to national jurisdiction— and national airspace), with suggestions ranging from about 30 km (19 mi) (the extent of the highest aircraft and balloons) to about 160 km (99 mi) (the lowest extent of short-term stable orbits). The Fédération Aéronautique Internationale has established the Kármán line, at an altitude of 100 km (62 mi), as the boundary between the Earth's atmosphere and the outer space, while the United States considers anyone who has flown above 50 miles (80 km) to be an astronaut; indeed descending space shuttles have flown closer than 80 km (50 mi) over other nations, such as Canada, without requesting permission first. Nonetheless both the Kármán line and the U.S. definition are merely working benchmarks, without any real legal authority over matters of national sovereignty.