/ Dictionary / Index A actinide: Meaning and Definition of Find definitions for: ac•ti•nide Pronunciation: (ak'tu-nīd"), [key] — n. Chem. any element of the actinide series. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: actinide (Thesaurus) actinic ray actinide series Related Content Daily Word Quiz: reclusive Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language