sea·bor·gi·um (s?-bôr?g?-?m)n. Symbol Sg An artificially produced radioactive element with atomic number 106 that has only been produced in trace amounts. The isotope with mass number 266 has the longest confirmed half-life (21 seconds), although heavier isotopes with longer half-lives have been reported. See Periodic Table.[After Glenn Theodore Seaborg.]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.seaborgium (?si?b?????m) n (Elements & Compounds) a synthetic transuranic element, synthesized and identified in 1974. Symbol: Sg; atomic no: 106[C20: named after Glenn Seaborg]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014sea·bor·gi·um (s?-bôr?g?-?m) Symbol Sg A synthetic, radioactive element that is produced by bombarding californium with oxygen ions or bombarding lead with chromium ions. Its most stable isotope has a half-life of about 20 seconds. Atomic number 106. 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.