military police

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

• noun The branch of an armed force assigned to perform law enforcement duties, as on a military installation.

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

• noun A unit of a military organization whose duty is to police that particular organization. They are in charge of law enforcement on military property, and of military personnel.

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

• noun a military corps that enforces discipline and guards prisoners