Wiktionary
n. (context isotope English) An isotope of uranium, used for energy generation and in atomic weapons, containing one hundred and forty-three neutrons. It has a half-life of 7·038×108 years, whereupon it decays into thorium-231.
Wikipedia
Uranium-235 (U) is an isotope of uranium making up about 0.72% of natural uranium. Unlike the predominant isotope uranium-238, it is fissile, i.e., it can sustain a fission chain reaction. It is the only fissile isotope that is a primordial nuclide or found in significant quantity in nature.
Uranium-235 has a half-life of 703.8 million years. It was discovered in 1935 by Arthur Jeffrey Dempster. Its (fission) nuclear cross section for slow thermal neutrons is about 584.994 barns. For fast neutrons it is on the order of 1 barn. Most but not all neutron absorptions result in fission; a minority result in neutron capture forming uranium-236.
Usage examples of "uranium-235".
In that one cigar-box-size block there is enough uranium-235 to stick under one fingernail without discomfort.
The Hanford plant on the Columbia River was striving to produce plutoniUM, u enough venture, it was a bright hope compared with the two colossal installations at Oak Ridge intended to separate uranium-235 by two different methods, both still sputtering and failing.
In those days they used both methods in complementary fashion and created enough uranium-235 to make Little Boy.
Even taking a worst-case scenario, they said—assuming not one but two entire gas diffusion centrifuge cascades operating not for one but for two years—there was no way in creation Iraq could have more than half the uranium-235 she would need for a single medium-yield device.