from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A radioactive, inert, gaseous isotope of radon, with a half-life of 3.92 seconds.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Any of the radioactive isotopes of radon
- n. Any radioactive isotope of an actinide
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. any of a series of radioactive elements with atomic numbers 89 through 103
actin(ium) + -on2.(American Heritage® Dictionary of the English Language, Fourth Edition)