Myelin \My"e*lin\, n. [Gr. ? marrow.]

(Physiol. Chem.)
   (a) A soft white substance constituting the medullary sheats of nerve fibers, and composed mainly of cholesterin, lecithin, cerebrin, albumin, and some fat.
   (b) One of a group of phosphorized principles occurring in nerve tissue, both in the brain and nerve fibers.

Source: Webster's Revised Unabridged Dictionary (1913)