A
chemical element, in the
periodic table uranium has the symbol U and atom number 92.
Radioactive element (it doesn't have stable isotopes).
Uranium was the first element that was found to be fissionable, i.e. upon bombardment with
slow neutrons, its U235 isotope becomes the very short lived U236, that immediately divides into two smaller nuclei, liberating energy
and more neutrons. If these neutron are absorbed by other U235
nuclei, a chain reaction occurs, and if there isn't anything to
absorb some neutrons and slow the reaction, it is explosive. The first atomic bomb worked with by this principle (fission). A more accurate
name for both this and the Hydrogen bomb (fusion) would be "nuclear
bomb", because only the nuclei participate.