has·si·um (hä?s?-?m)n. Symbol Hs An artificially produced radioactive element with atomic number 108 that has known isotopes with mass numbers ranging from 263 to 277, all of which decay by alpha particle emission or spontaneous fission. The isotope with the longest confirmed half-life (16.5 minutes) is Hs-277. See Periodic Table.[From Medieval Latin Hassia, Hesse (German state containing Darmstadt, where the element was first synthesized).]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.hassium (?hæs??m) n (Elements & Compounds) a synthetic element produced in small quantities by high-energy ion bombardment. Symbol: Hs; atomic no: 108[C20: from Latin, from Hesse, German state where it was discovered]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014has·si·um (hä?s?-?m) Symbol Hs A synthetic, radioactive element that is produced by bombarding lead with iron ions. Its most stable isotope has a half-life of two milliseconds. Atomic number 108. 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.