sky marshal

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

• noun An armed federal law-enforcement officer, usually working undercover on an airliner during a flight, assigned to prevent or stop air piracy and acts of terrorism involving commercial aircraft.

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

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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