ruth·er·for·di·um (r?th??r-fôr?d?-?m)n. Symbol Rf An artificially produced radioactive element with atomic number 104 that has only been produced in trace amounts. The isotope with mass number 261 has the longest confirmed half-life (65 seconds), although isotopes with longer half-lives have been reported. See Periodic Table.[After Ernest Rutherford.]American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.rutherfordium (?r?ð??f??d??m) n (Elements & Compounds) a transactinide element produced by bombarding californium-249 nuclei with carbon-12 nuclei. Symbol: Rf; atomic number.: 104; atomic wt: 261. Name in the former Soviet Union: kurchatovium [C20: named after Ernest Rutherford]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014un?nil?qua?di?um (?yu n?l?kw?d i ?m) n. provisional name for the transuranic element with atomic number 104. Symbol: Unq Also called rutherfordium. [1975?80;