/ Dictionary / Index P pineal apparatus: Meaning and Definition of Find definitions for: pin'eal apparat'us Pronunciation: [key] a median outgrowth of the roof of the diencephalon in vertebrates that in some develops into the pineal eye and in others into the pineal gland. See Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. pineal pineal body Related Content Daily Word Quiz: endear Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language