/ Dictionary / Index M mesencephalon: Meaning and Definition of Find definitions for: mes•en•ceph•a•lon Pronunciation: (mes"en-sef'u-lon", -lun, mez"-), [key] — pl. -la -lons. the midbrain. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: mesencephalon (Thesaurus) mesembryanthemum mesenchyme Related Content Daily Word Quiz: simian Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language