flag carrier

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An airline owned by or strongly identified with a nation.
• noun An airline or merchant vessel registered in a given country.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A person carrying a standard or flag, usually at sporting events and parades, previously on the battlefield.
• noun business A transportation company, such as a shipping or airline company, that is registered in a given state
• noun politics A strong supporter of a cause, or perhaps the strongest and most visible supporter of said cause.
• noun A national airline (typically having the flag, or other national insignia, on the tail)
• noun military, nautical An aircraft carrier serving as a flagship