When different countries negotiate agreements over where their respective
airlines can fly, the "freedoms of the air" are often cited. The first six were defined in the
International Air Services Transit Agreement of
1944, and are still used today, with a couple of additional freedoms thrown in
for good measure.
First freedom: The right to overfly a country without getting shot down.
Second freedom: The right to stop in a country for refueling or maintenance on the way to another. Years ago, European planes would stop at Shannon Airport on their way to the Americas: likewise, planes flying from Europe to East Asia would often stop in Anchorage, Alaska. Second freedom rights do not include the right to discharge or take on passengers or freight. Almost all countries in the world have granted each other first and second freedom rights.
Third freedom: The right to carry passengers from one's own country to another: e.g. for British Airways to carry British passengers to America.
Fourth freedom: The right to carry passengers from another country to one's own: e.g. for British Airways to carry American passengers to Britain. As you might guess, third and fourth freedom rights are almost always granted simultaneously in bilateral agreements between countries.
Fifth freedom: The right to carry passengers from one's own country to a second country, and from that country to a third country. For instance, United, Northwest, Thai, CAL, and Singapore Airlines all have fifth-freedom rights to take passengers from the United States to Japan, and from Japan to other Asian countries.
Sixth freedom: The right to carry passengers from a second country to a third country by stopping in one's own country. Cathay Pacific, for example, can ticket passengers from Australia to Europe with a connection in Hong Kong.
Seventh freedom: The right to carry passengers on direct flights between two foreign countries without continuing service to the airline's home country. This is much rarer than the first six freedoms, although it's not unheard of: Grupo TACA, for example, offers service from the United States and Canada to Cuba, which does not feed any of its other routes.
Eighth freedom: Also known as cabotage, this is the right for an airline to carry passengers on domestic routes within a foreign country. The European Union has established eighth freedom rights between all of its member states: otherwise, such rights are only granted in isolated instances where the domestic air network is very underdeveloped. A notable instance is Pan Am's authority to fly between Frankfurt and West Berlin during the 1950's and 1960's.