1) A fatty, axon-enwrapping sheath that serves to speed up neural conduction, formed by concentric layers of Schwann's cell (peripheral) or oligodendrocyte (CNS) membranes; loss or damage leads to severe loss of neural function, as in multiple sclerosis.

2) The lipid-rich sheath surrounding axons in both the central and peripheral nervous systems. The myelin sheath is an electrical insulator and allows faster and more energetically efficient conduction of impulses. The sheath is formed by the cell membranes of glial cells (Schwann cells in the peripheral and oligodendroglia in the central nervous system). Deterioration of the sheath in demyelinating diseases is a serious clinical problem.

3) The fatty substance that covers and protects nerves.