An artificially produced radioactive element with atomic number 104 whose most long-lived isotopes have mass numbers of 253, 255, 257, and 259 with half-lives of 1.8, 1.6, 4.7, and 3.4 seconds, respectively. See Table at element.
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.