Fermium, n.
the transuranic element of atomic number 100; symbol Fm. The atomic weight of the most stable isotope, having a half-life of about 80 days, is 257. The first isotope, Fm255 was discovered in 1952 in the debris of a thermonuclear explosion. Other istopes have been produced in nuclear reactors and by decay of other transuranic elements. [WordNet 1.5]