muscle fiber

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

• noun A cylindrical, multinucleate cell composed of numerous myofibrils that contracts when stimulated.

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

• noun A single cell of a muscle. Each muscle fibers contain many myofibrils, the contractile unit of muscles.

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

• noun an elongated contractile cell that forms the muscles of the body