Meitnerium is a synthetic chemical element; it has symbol Mt and atomic number 109. It is an extremely radioactive synthetic element. The most stable known isotope, meitnerium-278, has a half-life of 4.5 seconds, although the unconfirmed meitnerium-282 may have a longer half-life of 67 seconds. The GSI Helmholtz Centre for Heavy Ion Research near Darmstadt, Germany, first created this element in 1982. It is named after Lise Meitner.

The GSI Helmholtz Centre for Heavy Ion Research is a federally and state co-funded heavy ion (Schwerion) research center in the Wixhausen suburb of Darmstadt, Germany. It was founded in 1969 as the Society for Heavy Ion Research, abbreviated GSI, to conduct research on and with heavy-ion accelerators. It is the only major user research center in the State of Hesse.