sky marshal

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. An armed federal law-enforcement officer assigned to prevent and interdict air piracy and acts of terrorism involving commercial aircraft.

Wiktionary

1. n. US, law enforcement An armed, undercover guard on a commercial-airline flight.
2. n. fiction A military rank in fiction for air force, space force, or armed forces, usually the topmost rank

WordNet 3.0

1. n. a person trained by the government in hijacking and terrorist tactics who (for security reasons) is a passenger aboard an airline flight