/ Dictionary / Index A actinium series: Meaning and Definition of Find definitions for: actin'ium se"ries Pronunciation: [key] — Chem. Chem. the radioactive series that starts with uranium 235 and ends with a stable isotope of lead of mass number 207. Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease. actinium emanation actino- Related Content Daily Word Quiz: digression Analogy of the Day: Today’s Analogy Spelling Bee: Today’s Spelling Bee Frequently Misspelled Words Frequently Mispronounced Words Easily Confused Words Writing & Language