/ Dictionary / Index F fragile X syndrome: Meaning and Definition of Find definitions for: fragile X syndrome Pronunciation: [key] a widespread form of mental retardation caused by a faulty gene on the X chromosome. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. fragile fragment Related Content Daily Word Quiz: gregarious Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language