air marshal

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A security officer who travels undercover on a commercial airliner to prevent hijacking.

Wiktionary

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

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