dub·ni·um (do?ob?n?-?m)n. Symbol Db An artificially produced radioactive element with atomic number 105. The isotope with mass number 262 has the longest confirmed half-life (35 seconds), although longer half-lifes have been reported for some of its heavier isotopes. See Periodic Table.[After Dubna, Russia, location of the United Institute of Nuclear Research.]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.dubnium (?d?bn??m) n (Elements & Compounds) a synthetic transactinide element produced in minute quantities by bombarding plutonium with high-energy neon ions. Symbol: Du; atomic no 105. See hahnium[C20: after Dubna, where it was first reported]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014dub·ni·um (do?ob?n?-?m) Symbol Db A synthetic, radioactive element that is produced from californium, americium, or berkelium. Its most stable isotope has a half-life of 34 seconds. Atomic number 105. See Periodic Table.The American Heritage® Student Science Dictionary, Second Edition. Copyright © 2014 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.