pressure group

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

• noun An interest group that endeavors to influence public policy and especially governmental legislation, regarding its particular concerns and priorities.

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

• noun A group of people who try to influence the policies of governments and other legislative organizations

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

• noun a group of people who try actively to influence legislation