/ Dictionary / Index M myelin: Meaning and Definition of Find definitions for: my•e•lin Pronunciation: (mī'u-lin), [key] — n. Biol. a soft, white, fatty material in the membrane of Schwann cells and certain neuroglial cells: the substance of the myelin sheath. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: myelin (Thesaurus) myelencephalon myelinated Related Content Daily Word Quiz: immense Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language