/ Dictionary / Index A actinism: Meaning and Definition of Find definitions for: ac•tin•ism Pronunciation: (ak'tu-niz"um), [key] — n. the property of radiation by which chemical effects are produced. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. See also: actinism (Thesaurus) actinin actinium Related Content Daily Word Quiz: agitated Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language