myelin sheath

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

• noun The insulating envelope of myelin that surrounds the core of a nerve fiber or axon and facilitates the transmission of nerve impulses. In the peripheral nervous system, the sheath is formed from the membrane of the Schwann cell, and in the central nervous system it is formed from oligodendrocytes.

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

• noun neuroanatomy An envelope of myelin that surrounds nerve fibres.

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

• noun a layer of myelin encasing (and insulating) the axons of medullated nerve fibers