Nuclear fission was a nuclear reaction in which a heavy nucleus splits into two lighter nuclei, releasing a considerable amount of energy.
In the 20th century, nuclear fission was used as a power source in nuclear fission reactors, but also as a devastating weapon. Nuclear fission produced toxic side effects, including beta radiation. (Star Trek IV: The Voyage Home; Star Trek: First Contact; DS9: "Little Green Men")
Nuclear fission reactors were abandoned at the start of the fusion era, their use rated a "dubious flirtation". (Star Trek IV: The Voyage Home)
The thought of nuclear fission taking place in a planet's atmosphere shocked Quark and Rom when they learned about it from Nog in 1947. (DS9: "Little Green Men")