splinter group

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

• noun A group, such as a religious sect or political faction, that has broken away from a parent group.

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

• noun A group of members of a political party or of a similar organisation who have decided to create their own organisation.

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

• noun a faction or sect that has broken away from its parent organization