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a radioactive silver-white metallic element that glows blue in the dark, resembling the rare earths in chemical behavior and valence. Symbol: Ac; atomic number: 89; atomic weight: 227.
Historical Examples

Other radio-active substances have each several derivatives: actinium has nine, uranium has four.
Inventors at Work George Iles

actinium, an element or elementary substance obtained in minute quantities in connection with the study of radioactivity.
The New Gresham Encyclopedia. Vol. 1 Part 1 Various

For if actinium was an independent element we would be sure to find actinium haloes.
The Birth-Time of the World and Other Scientific Essays J. (John) Joly

In 1902 Giesel discovered another substance which he called emanium, and which was considered to be identical with actinium.
The New Gresham Encyclopedia. Vol. 1 Part 1 Various

Emanium was the name originally proposed by Giesel for the body now known as actinium.
A Brief Account of Radio-activity Francis Preston Venable

M. Debierne has obtained a third radioactive substance from pitchblende, which he has called “actinium.”
Alchemy: Ancient and Modern H. Stanley Redgrove

The element has since been found in nature as a small constituent of the natural decay of actinium.
A Brief History of Element Discovery, Synthesis, and Analysis Glen W. Watson

These are generally known as radium emanation, thorium emanation, and actinium emanation.
A Brief Account of Radio-activity Francis Preston Venable

On Rutherford’s theory then, actinium should always accompany uranium and radium, but in very subordinate amount.
The Birth-Time of the World and Other Scientific Essays J. (John) Joly

In this class are now placed also the three radio-active gases, emanating respectively from radium, thorium, and actinium.
A Brief Account of Radio-activity Francis Preston Venable

noun
a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no: 89; half-life of most stable isotope,227Ac: 21.6 years; relative density: 10.07; melting pt: 1051°C; boiling pt: 3200 ± 300°C
n.

radioactive element discovered in 1899, from Greek actin-, comb. form of aktis (genitive aktinos) “ray, radiance” (see actino-) + chemical suffix -ium.

actinium ac·tin·i·um (āk-tĭn’ē-əm)
n.
Symbol Ac
A radioactive element found in uranium ores. Its longest lived isotope is Ac 227 with a half-life of 21.6years. Atomic number 89; melting point 1,050°C; boiling point (estimated) 3,200°C; specific gravity (calculated) 10.07; valence 3.
actinium
(āk-tĭn’ē-əm)
Symbol Ac
A silvery-white, highly radioactive metallic element of the actinide series that is found in uranium ores. It is about 150 times more radioactive than radium and is used as a source of alpha rays and neutrons. Its most stable isotope has a half-life of about 22 years. Atomic number 89; melting point 1,050°C (1,922°F); boiling point (estimated) 3,200°C (5,792°F); specific gravity (calculated) 10.07; valence 3. See Periodic Table.