The story
From uranium + 235 (the atomic mass of the isotope).
Etymology from Wiktionary, CC BY-SA 4.0.
uranium-235
Today it means: A fissile isotope of uranium, used for energy generation and in atomic weapons, containing 143 neutrons. It has a half-life of 7.038×10⁸ years; it decays into thorium-231 and ultimately lead. Full definition →