meit·ner·i·um (m?t-nûr??-?m)n. Symbol Mt An artificially produced radioactive element with atomic number 109 that has known isotopes with mass numbers ranging from 265 to 279. The isotope with the longest confirmed half-life (0.7 seconds) is Mt-268. See Periodic Table.[After Lise Meitner.]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.meitnerium (?ma?tn??r??m) n (Elements & Compounds) a synthetic element produced in small quantities by high-energy ion bombardment. Symbol: Mt; atomic no: 109[C20: named after Lise Meitner]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014meit·ner·i·um (m?t-nûr??-?m) Symbol Mt A synthetic, radioactive element that is produced by bombarding bismuth with iron ions. Its most stable isotope has a half-life of 70 milliseconds. Atomic number 109. 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.