vigilance committee

from The American Heritage® Dictionary of the English Language, 4th Edition

• n. A volunteer group of citizens that without authority assumes powers such as pursuing and punishing those suspected of being criminals or offenders.

from the GNU version of the Collaborative International Dictionary of English

• n. a volunteer committee of citizens for the oversight and protection of any interest, esp. one organized for the summary suppression and punishment of crime, as when the processes of law appear inadequate.

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

• n. a volunteer committee to maintain order where an efficient legal system does not exist