/ Dictionary / Index M mononucleosis: Meaning and Definition of Find definitions for: mon•o•nu•cle•o•sis Pronunciation: (mon"u-n"klē-ō'sis, -ny"-), [key] — n. Pathol. the presence of an abnormally large number of mononuclear leukocytes, or monocytes, in the blood. See Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: mononucleosis (Thesaurus) mononuclear monopetalous Related Content Daily Word Quiz: panacea Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language