task force

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

• noun A temporary grouping of military units or forces under one commander for the performance of a specific operation or assignment.
• noun A temporary grouping of individuals and resources for the accomplishment of a specific objective.

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

• noun idiomatic A group of people working towards a particular task, project, or activity, especially assigned in a particular capacity.

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

• noun a temporary military unit formed to accomplish a particular objective
• noun a semipermanent unit created to carry out a continuing task