/ Dictionary / Index N Neptunian: Meaning and Definition of Find definitions for: Nep•tu•ni•an Pronunciation: (nep-t'nē-un, -ty'-), [key] — adj. pertaining to Neptune or the sea. pertaining to the planet Neptune. (often l.c.)formed by the action of water. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. neptune neptunium Related Content Daily Word Quiz: dissonant Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language