air marshal

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

• noun A security officer who travels undercover on a commercial airliner to prevent hijacking or terrorist activity.

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

• noun US, law enforcement An armed, undercover guard on a commercial-airline flight.
• noun military A military rank used by air forces, mostly in Commonwealth countries.

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