The Collaborative International Dictionary of English v.0.48:

myelin \my"e*lin\, myeline \my"e*line\, n. [Gr. myelo`s marrow.]
   (Physiol. Chem.)
   (a) A soft white substance constituting the medullary sheaths
       of nerve fibers, and composed mainly of cholesterin,
       lecithin, cerebrin, protein, and some fat.
   (b) One of a group of phosphorized principles occurring in
       nerve tissue, both in the brain and nerve fibers.
       [1913 Webster]

WordNet (r) 3.0 (2006):

myelin
    n 1: a white fatty substance that forms a medullary sheath
         around the axis cylinder of some nerve fibers [syn:
         myelin, myeline, medulla]