from The American Heritage® Dictionary of the English Language, 4th Edition
- n. Any of a series of chemically similar, radioactive elements with atomic numbers ranging from 89 (actinium) through 103 (lawrencium).
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Any of the 14 radioactive elements of the periodic table that are positioned under the lanthanides to which they have similar chemistry.
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) + -ide.(American Heritage® Dictionary of the English Language, Fourth Edition)
actin- + -ide (Wiktionary)