flag carrier

Short definition from Wiktionary

Noun

1. 1
   A person carrying a standard or flag, usually at sporting events and parades, previously on the battlefield.
2. 2
   A strong supporter of a cause, or perhaps the strongest and most visible supporter of said cause.
3. 3
   A transportation company, such as a shipping or airline company, that is registered in a given state.
4. 4
   A national airline (typically having the flag, or other national insignia, on the tail).
5. 5
   An aircraft carrier serving as a flagship.