Definitions

from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. A radioactive element found in uranium ores, used in equilibrium with its decay products as a source of alpha rays. Its longest lived isotope is Ac 227 with a half-life of 21.6 years. Atomic number 89; melting point 1,050°C; boiling point (estimated) 3,200°C; specific gravity (calculated) 10.07; valence 3. See Table at element.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. a radioactive, metallic chemical element (symbol: Ac) with an atomic number of 89; found in uranium ores

from the GNU version of the Collaborative International Dictionary of English

  • n. A supposed metal, said by Phipson to be contained in commercial zinc; -- so called because certain of its compounds are darkened by exposure to light.

from The Century Dictionary and Cyclopedia

  • n. A supposed chemical element found associated with zinc. Its chemical and physical properties have not been fully investigated.
  • n.
  • n. A radioactive substance found by Debierne to exist in the residues remaining from pitchblende after the extraction of the uranium: a new radio-element closely related in its chemical behavior to lanthanum, from which it has not as yet been found possible to separate it completely.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. a radioactive element of the actinide series; found in uranium ores

Etymologies

From Greek aktīs, aktīn-, ray (from its radioactivity).
(American Heritage® Dictionary of the English Language, Fourth Edition)
actin- + -ium (Wiktionary)

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