Plural: actinia

ETYM Greek aktis, aktin ray.
Rare radio-active element.
White, radioactive, metallic element, the first of the actinide series, symbol Ac, atomic number 89, atomic weight 227; it is a weak emitter of high-energy alpha particles.
Actinium occurs with uranium and radium in pitchblende and other ores, and can be synthesized by bombarding radium with neutrons. The longest-lived isotope, Ac-227, has a half-life of 21.8 years (all the other isotopes have very short half-lives). Actinium was discovered in 1899 by the French chemist André Debierne.
A radioactive element of the actinide series; found in uranium ores; SYN. Ac, atomic number 89.