/ Dictionary / Index A actinin: Meaning and Definition of Find definitions for: ac•ti•nin Pronunciation: (ak'tu-nin), [key] — n. Biochem. a contractile protein of muscle. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. actiniform actinism 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