myelin

n. fatty substance which surrounds nerve fibers (Anatomy)

Myelin is an electrically insulating phospholipid layer that surrounds the axons of many neurons. It is an outgrowth of glial cells: Schwann cells supply the myelin for peripheral neurons while oligodendrocytes supply it to those of the central nervous system. Myelin is considered a defining characteristic of the (gnathostome) vertebrates, but it has also arisen by parallel evolution in some invertebrates. Myelin was discovered in 1878 by Louis-Antoine Ranvier.
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Myelin /my·elin/ the lipid-rich substance of the cell membrane of Schwann cells that coils to form the myelin sheath surrounding the axon of [more]Myelin - Community and Resources

Noun
1. a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers
(synonym) myeline, medulla
(hypernym) fat
(substance-holonym) medullary sheath, myelin sheath

n. myelin, fatty substance which surrounds nerve fibers (Anatomy)