rotator cuff

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

• noun An anatomical structure composed of the tendons of four muscles that control rotation of the shoulder and of parts of the capsule of the shoulder joint.
• noun The set of four muscles that control rotation of the shoulder.

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

• noun A set of four smaller muscles in the shoulder responsible for rotating the humerus (upper arm bone).

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

• noun a supporting structure of the shoulder consisting of the muscles and tendons that attach the arm to the shoulder joint and enable the arm to move