Atomic number 89 refers to the chemical element actinium, which is a rare, silvery-white, radioactive metal. Synonyms for this term include Actinon, Actinium-225, and Ac-225. Actinium is widely used in medical treatments and research, particularly in radiation therapy for cancer, due to its high energy and short half-life. Its radioactive properties also make it useful in geochronology and nuclear fission reactors. However, its rarity and difficulty in obtaining make it an expensive and limited resource. Despite its importance and potential benefits, actinium and its synonyms are not commonly known terms outside of scientific and medical circles.