fermium

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A synthetic transuranic metallic element (atomic number 100) having 10 isotopes with mass numbers ranging from 248 to 257 and corresponding half-lives ranging from 0.6 minutes to approximately 100 days. See Table at element.

Wiktionary

1. n. A transuranic chemical element (symbol Fm) with an atomic number of 100.

GNU Webster's 1913

1. 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 3.0

1. n. a radioactive transuranic metallic element produced by bombarding plutonium with neutrons