The Collaborative International Dictionary of English v.0.48:

fermium \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. --Handb.
   Ch. Phys. 61.

   Syn: Fm.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

fermium
    n 1: a radioactive transuranic metallic element produced by
         bombarding plutonium with neutrons [syn: fermium, Fm,
         atomic number 100]